is there any harm in having a seatpost that's really tall, on a singlespeed MTB? i have the option to get a frame that'll be a few inches small for me for cheap.

lots of exposed seat post, no.

the only harm in riding a small frame is having a short top tube, which will force you into weird positions, cramping your body and will probably make you quite uncomfortable after a while.

well, loooking at the specs online, the TT length difference is only 1.25 inches, which doesnt seem like that much. couldnt that be dealt with by getting a diff. stem?

Sure but that messes with the handling... or so they say. If you used to have a short stem (say, 90mm) you might as well give it a shot with a 110 or 120.

my ss/fg mtb frame is a little small for me, i run around with the seat jacked up pretty high on it and run a fairly short stem (i think its a 90) and have no cramping issues at all... i just kept the bars down low (0 degree rake on the stem, and only one short spacer on the steerer) and i mounted some bar extenders just to the inside of the brake lever(s) giving me a nice comfortable riding position most of the time when i dont need alot of torque...

i've had wierd nasty cramping sensations riding bikes fitted to other people... even family members that are not sized that terribly different from me... but i must say my kona never let me down in the comfort department and i really like the ample ball room when snow and trail riding
-pete