hi kate ... i did my fist sprint tri on a mountain bike and loved it. The training I had put in seriously paid off letting me whoop past alot of those "really cool bikes" (as you called them). In fact if honest most of us will have started with a non-cool bike. It aint the bike that makes the speed its the engine. Then if you do really get into tri ..which we have no doubt you will then later invest in a tri / road bike as well as all the the other kit you will end up getting... another excuse for shopping ... its so good madnurse
That sounds like a plan - I'll be the one with the crap bike - only joking!!!!