So I should soon be taking delivery of my new commuter!! Wheee.

It's pretty much a flat bar roadie that comes with 700x28 tyres. One of the first things I'll be doing is swapping out the tyres to something more flat resistant. Probably a set of Gatorskins at this point.

So first off, I don't have the specific details of the rims, but considering they come with 700x28 tyres, what is the likely range of tyre width I could fit on?

Secondly, what width tyre should I go with as a replacement?

Currently I have a roadie with 700x23 tyres so I'm used to riding with those. I wouldn't mind a slightly wider tyre for the bumps etc during commuting but I don't want to sacrifice too much speed. So can anyone comment on tyre width in general?

Or any other tyre recommendations?

How far is your commute.
What are the roads like (gradient, surface quality).
How much load do you carry (everyday and max shopping load).

Commute is actually 30Km each way but with the option of a partial ride or only going one way a day (good bike facilities at work). I want to regularly ride at least one way rather than do the partial ride.

Roads are mostly OK, can do it on the roadie. Although there's something that doesn't quite feel right when riding over small seeds as they go pop pop pop-pop out from under the tyres.
Gradient - Well I take the longer way to avoid hills

Everyday load for work - bike stuff (spare tube, tools etc.) change of clothes, lunch, wallet, keys. Have Topeak Pannier bag and will fill the top bit and most of one side.
Max shopping load - Don't plan to use for shopping. Have supermarket down the road so I walk.

The rim should handle 25-32 without any problems. It depends how much clearance you have at the brakes and frame. 28mm is a good size for everyday, sporty use but note that the actual width of a 28mm varies greatly from brand to brand.

What is the bike? will the frame even take anything larger then a 28?

Room needed for fenders?

any option of leaving a few changes of clothes (including shoes)/ lunch making materials/ etc at your workplace?

+1. 28mm is a sweet spot for me, anything larger and the bike feels like a beach cruiser.

A light 32 can sometimes work, but more than that and it's not really a road bike. Something around 30 would be great, and luckily a lot of 32 measure a good bit smaller.

My road bike currently has 25s (Bonetrager RaceLites) and I decided to go with 28s (Schwalbe Marathon Supremes) on my winter commuter cyclocross bike for my 20 mile round trip commute. I don't notice much of a loss in speed and they seem to be a little softer and more forgiving with better traction in the wet. Although the Supremes do kick up a lot more spray.

How does exceeding 32mm tire width turn a road bike into something else? What does it then become?

I agree... I must be a fool for not noticing that much difference between a 35 mm studded winter tire and a 28 mm Gatorskin. Of course other than more comfort and noise on the studded. I have been tracking average speed for a long time and don't really find much of a difference in anything. Flatbar vs drops or wide vs skinny... I did just change from a 28 mm Gatorskin to a 28 mm Continental Touring Plus and the Conti is far harsher (hardcase though). But really, when it comes to speed, my legs make all the difference in the world. I say go with what is most practical. I really like Hardcase tires (Race Lite) as I have never had a flat on them. My Gatorskins however do tend to flat more and they seem to rip too. I just lost a tire that likely had less than 2000 km on it due to a rip in the tire that went through the kevlar or whatever it is.

Well, wider tires have less rolling resistance and they are more comfortable so I would go with the biggest size you can stuff in there as well as fenders.

That said, my commuter runs 28mm with fenders. I would go wider if I could.

Look at some of those cyclists rolling on 42mm 650B hetres. Some of them still blaze. A lot of tires 32mm and up are lower quality, hybrid tires, which can account for the sluggishness in changing sizes. Go with a high quality tire (or any tire that is comparable to the 23mm you are riding) and you will notice an improvement.

A tourer.

Yes, when you put fatter tires on a road bike, the chainstays tend to elongate, eyelets form, and angle start to shift.

Cyclists beware.

So the only difference between a touring bike and a road bike is tire width?

Patience , grasshopper, after it gets there you can measure,
the space available.

Tire recommendations: 25 or 28 mm Schwalbe Marathon Plus. Increased rolling resistance, but really superior flat resistance. Other brands/models may be just as good or better, however this product has worked for me very well for over 4 years.

My commuter rig is a 2 bike system. A multi-geared 'loaded' 2x9 CX bike w/rack, trunkbag, panniers,full coverage fenders etc. and a fg w/a seatpost rack/trunkbag combo and SKS Raceblade fenders. Both have a Niterider Mi-Newt cordless 150, Sette Glo HL and 3 Sette 316 rear blinkies. The fg and the multi-geared have 25 and 28 mm SMPs, respectively.

As you increase tire size (all other things being equal) you increase the amount of weight. The weight in and of itself isn't a lot, but it's rotational weight so it very much affects acceleration/spin-up. Not so important if you're commuting or touring but if you're racing, the extra weight equates to a lot of extra wasted energy over the course of a long race.

Also, it's not as simple as saying that a wider tire has less rolling resistance, it really depends on the surface. A super skinny tire inflated to max pressure has the lowest rolling resistance if the riding surface is exceptionally smooth (track racing or insanely well maintained roads and bike paths). The optimal tire really depends on where/what surface you ride on.

I think this is what I'll end up doing.

At this stage I am leaning towards a 25 mm. Because it is a tad wider than the 23 but also I will be able to use the same inner tubes for roadie and commuter. I bought a bulk pack so the idea is I use the same stock. 28mm and above I believe generally require a different tube.

Shameless plug for sure, but if you bought Schwalbe Tubes, an SV 15 fits 700x18 - 700x28. Just sayin'.

I have been riding on 700x35's for about two years and I wouldn't go any narrower. From my perspective, the 35's provide a very nice ride but are still plenty fast for me. Of course, the speed really has much more to do with the motor than the wheels...

25 for rain.
23 for sun.
1.5 for ice and snow.

I know about touring but could you please explain the difference between commuting and racing.

I'd go with 28 or 32, if you're going 30km each way and since they come with 28s. I keep separate sets of tubes for all my bikes, since my road bike is 23s, my hybrid varies from 25-32, and my MTB bike varies from 26x1.25-26x2.0.

Commuting, you win if you get to work, and get home again. Racing, you only win if you're first to the finish line. Over the course of say an hour-long crit or a 2-4 hour road race every acceleration takes a certain amount of energy. Racing is all about being in a position to try and win a race while conserving as much energy as possible so that you have more left in the tank than your competitors when that inevitable final acceleration occurs.