25x1.0 slick tyre on MTB.. or width < 1.5
 

25 x1.0 is the narrowest I've seen. I'm considering a purchase and hope someone has experience they can share. For commuting and exercise.

1.35 (slick) was plenty narrow enough for my wife's MTB commuter.

Ritchey Tom Slicks come in 1.1, but that's really narrow. I'd honestly recommend staying away from them, they give a really harsh ride.

I run 26x1 1/8 gatorskins...I like them a lot!

Specialized Fatboys 26*1.25 and up to 100psi. Since my other tires on the bike were different (original knobbies to Michelin transworld city's) it's a world of difference. They are lighter and roll really well IMHO. I got more flats when they were new but nothing that I can remember over the last 5-6 months.

can I expect flat tires w/ greater frequency? If so would that be a rim tape issue or they just don't have knobs to absorb an occasional glass shard.

& I need new tubes? or can get away with mtb tubes

You will definitely need narrower tubes. If you have very wide rims (probably not, unless it's a downhill bike) then you would need narrower rims as well. Rim tape covers the spoke heads and prevents them from pushing into the tubes. You're probably thinking of tire liners which may help but will also add weight. I've heard mixed reviews of these, most people seem to think they are not worth it. Tire belt thickness will help a bit with the flats, but your best bet is to simply avoid as many objects as you possibly can.

One question for you though, why are you looking for a tire so narrow? Certainly, they will run faster but as Fuzz mentioned you'll have a much harsher ride. Fatter tires will give you a softer ride and better handling. Many of them, like Schwalbe Big Apples even have pretty low rolling resistance.

thinking of doing a duathlon on my mtb. I'm convinced road tires are faster. I guess I want something narrow enough to be compromise btwn commute and race.

I run the narrow Gatorskins (26" x 1 1/8", 155 psi), and love them. I highly recommend them. They have been reasonably resistant to punctures (although they still flat from time to time -- usually in response to significant road-junk).

Before the Gatorskins, I had Specialized Fatboys. I was astonished at how much better the Gatorskins rode in comparison to the Fatboys. The Gatorskins are higher pressure, but for some reason just *feel* better.

I cannot say for certain whether the high-pressure, narrow tires make me faster. I feel faster. But last year when I switched to some WTB Slickasaurus tires (1.75" 80 psi, very slight tread) for a few weeks, my average speed was not appreciably different.

commuting on 2.0 slicks was good for me. it was good to have all that meat for all those horrible roadways. if you're going to such a narrow tire for commuting then maybe you should get a bigger wheeled bike for the road. that's what I eventually did. I started with the MTB but then realized that a bigger faster wheel was a better idea. it sure is a lot less work!

You're right that they would be faster, in general but the situation in which you'll notice the biggest change is climbing. If you're going for straight speed, I would go as narrow as possible (26 x 1.0") though you certainly couldn't do wrong with a larger tire. I'm currently running 26 x 1.75" Michelin Country Rocks which may be a bit wider than you'd prefer, but they are a nice compromise between speed and comfort.

Primo Comets are 1.5"/100psi and are fairly light and roll faster than their measured size.

Flats can sometimes be a tricky thing to gauge. I tend to get more flats around April - June and then the occasional ones (sometimes) afterwards. Part of it is due to the debris that builds up over the winter that has yet to be cleaned up. Part of the problem was also me not paying attention to what's right on the road in front of me or cleaning off the tire after my commute (picking off glass shards or other debris which could work itself through). I've also noticed that my tires don't seem to collect as much debris now then they did when they were new.

I like my tires and only remember paying $20 at a LBS if memory serves me correctly. I might take a look at what Mad Scientist suggested once mine wear out.