document updated 18 years ago, on Nov 2, 2007

AGH. 63xc.com is one of the few sites that discusses making a fixed gear work on a bike with vertical dropouts. It's a PAIN IN THE ASS. It involves black magic and trial and error. But it sounds like it can usually be done.
SheldonBrown also has some fixed/MTB advice
good description of all the different kinds of dropouts
or, if you're more adventurous and can find a local welder, you can actually just weld horizontal dropouts on any bike...