This article is from the Triathlon FAQ, by with numerous contributions by James Goddard others.
You don't need a triathlon bike to do triathlons. Modified road bikes
are very common in triathlons. If you already own a road bike or plan
on doing other types of riding you may be better off with a road bike
with clamp on aerobars. The advantages of a triathlon bike are in the
posisitioning. They are setup to keep you more comfortable when in the
aero bars and to work the quads less, saving them for the run. Often
triathlon bikes are more aerodynamic than road bikes.