Bicycle Mechanics - Are 29er wheelsets the same as 700c wheelsets?

I know the rims are 700c, but is the rim diameter the same as a road bike 700c wheelset?

I have a really heavy 700c disc wheelset that I want to replace, but typically 700c disc wheelsets are pretty expensive. It seems that there are many more 29er disc wheelsets out there so the prices are better. I just want to insure that a road tire such as a 28mm will fit the rim without the rim being too wide (made for a 2" tire). There is also the rear wheel spacing to deal with too. Not sure what my current spacing is, probably 130 which would be another issue.

Hmm, I'm not sure this money-saving plan will work out. I assume you meant to ask about rim width (not diameter) and inner width, at that, given your (legit) concern about tire-rim fit. I generally try to stick with these timeless guidelines to determine whether a rim is too wide/narrow for a given tire:

http://www.sheldonbrown.com/tire-sizing.html#width

I expect it'd be dangerous or impossible to fit a 28 mm tire on a rim made for 2" (~50 mm) tires.

When I had 23mm tires on a 22mm rim (touring). The tire went on, held to the rim no problems. But lots of pinch flats.

Have you thought of getting a rim and having a shop build it onto your disc hub?

I've got a friend who has a 29er mountain bike with Bontrager rims and disc compatible hubs. I'm not sure what the width of the rims are, but the bike came with 29"x 2.2" tires, and she's ridden thousands of road miles with Conti 700 x 28c Ultra Gatorskins installed on the rims with no problems. Pre-built wheelsets intended for 29" mountain bikes will have 135mm hub spacing.

Keep in mind that disc hubs are MTB hubs pretty much all the time. That means 135mm axle spacing vs 130. That won't be an issue as long as your bike currently has an MTB stle hub in it already.

The 29er wheels all seem to come with slightly wider rims. I just got a set for my own Double Cross build I'm starting on and the rims are Mavic 719's that take tires from 28 to "as wide as will fit the frame". But all the options for brake hubs used these slightly wider rims. The 719's being 580'ish grams each. If you truly want a rim that'll do the smaller road size tires you'll have to have a shop make them up for you. I did a set like that years ago when I made up a Speciallized M4 MTB to be my own foul weather super commuter. Narrow Mavic roadie rims to XT hubs running disc brakes.

I'm pretty ignorant about road bike stuff, but I've heard (read) it mentioned about people not having enough room to fit a slightly larger tire because of brake clearance. Might that be a problem with a wider rim???
EDIT: Ignore me. I forgot the "disc" part!

Very much so on the racing road bikes. Next time you're in an LBS with some racing roadie bikes look at the brake calipers. They are short arm styles and hug closely to the wheel and tire. But if you look past at the fork and stays you won't see much more room for things anyway.

This is why cyclocross bikes or "hybrid" style frames make better commuters since they have room for fenders and slightly larger tires. You generally see that style of bike set up with canti or V brakes.

In this case though he's got a disc wheelset. I'm guessing from this that he's got a hybrid or cyclocross frame set. I haven't seen any pukka racing bikes that are willing to suffer with the weight of a disc system.