They are only illegal for UCI racing. They are not illegal for USCF races until 2007.

They are also legal for all triathlons except ITU elite/pro Olympic distance races.

I have one (Powerwing 700) set up for triathlons. I love it. I have a lot of flexibility in how I can set it up. I can go steep and low for short course racing. Or move the saddle back and put a few headset spacers in and go slack and not-so-low for long course. It is a really aerodynamic frame. And it is comfortable which really helps with the run that I have to do right after.