12 Y.O. Litespeed MTB, IRO Jamie Roy fixie, Custom Habanero Ti 'Cross, No name SS MTB, Old school lugged steel track bike (soon)
I know some of the guys I ride with run very narrow knobbies on their MTB's for racing. I'd say less than 1.5" wide-close, but not as narrow as a big cross tire. I can't remember who made them though-maybe WTB.
Yeah, just to echo R6G, with a little more specificity. Hutchinson makes a 26x1.3" tire, which is to say 650x32 or33mm. The tire is called the Cross Comp.
CyclocrossWorld.com sells them for 40 bones, but is currently out of stock. A Froogle search turned some up at fullcycles.com for $37.77 each. I think I got mine from Speedgoat, if I recall correctly, but I no longer see them there.
BTW, I really like mine--I use them for both Short Track XC, and the occasional cross race thet is technical enough to warrant using my hardtail.