basically, tt and tri bikes are the same. the difference, if there is any, is in how steep you ride the bike. triathletes like to ride steep - seat tube angle about 76 degrees or more (up to 80) - so the transition to the run is easier. most pro roadies like to keep the seat tube angle close to their regular road bike angle, about 73 degrees, so they don't have to get accustomed to a different fit. in keeping with this practice, the trek tt bike from a year or so ago had a set tube angle of 74 degrees as opposed to most other tt/tri bikes which have steeper seat tube angles. my tt bike is now set up at about 76 - 77 degrees, down from almost 80 last year, while my road bike is 73. but that's just me and your needs will probably differ.
there's some good info in the article referenced above if one takes the time to read it. tom demmerly knows quite a bit about cycling and equipment.