Originally Posted by chephy
Oh, I see, as opposed to, say, weight. Thanks for the clarifications.
Not at all. Tri/TT bikes are plenty light. It is not oppossed to anything necessarily. My guess is that traditional round'ish tubes are just cheaper and easier to design etc. Also, the rounder tubes are likely better "overall". Most standard type racing bikes are designed for a variety of riding types, uphill grinds, windy downhill bombs, etc.
Time trials and Tri's are often more long stretches, straighter etc.
Look at the bikes from Cervelo. A Road racing bike with aero tubing
http://www.cervelo.com/bikes.aspx?bike=SLC2006
Of course I am neither an expert at bike design or racing, so this is just my best guess.
-D