As one of the resident Triathletes on here I'll weigh in...
I currently ride a Specialized Allez Dolce road bike w/ clip on aerobars. I've done all my shorter races on it and it's worked fine. That being said I'm now in the market for a tri specific geometry bike.
Basically a Tri bike is built w/ the purpose of dumping you down into the aerobars. You will hear people talk about how "steep" the bike is and they are referring to how steep the seat tube angle is or how far it "dumps" you forward on the bike. Tri bikes can seem squirrely when you first start riding them and if they are not known as good climbing bikes. Alot of bike clubs tend to look down on people w/ tri bikes because being down in the a-bars is not safe in a peloton situation. So group rides like that are pretty much out unless there are alot of other triathletes in the club as well. Some clubs are more "tri bike friendly" than others...
Road bikes are generally good all around bikes, and I would recommend that as a starting point. The one thing I've noticed w/ road bikes is they just really aren't made to be down in the aerobars for long periods of time. So if you plan on going long or always being in the aerobars you'll probably want a tri rig. But to start, I'd say road bike. Just my .02... Hope it helps. Feel free to drop me an email if you have any questions. I have years of triathlons under my belt and am currently training for my first Ironman.