Hmm, I thought there was one model that was. Guess not, since I can't find it. I'm afraid I'll have to go against getting a Tiperrary , no matter how comfortable they are, as there's no point in having a vest that isn't certified to protect you.
I agree with the make sure its certified comments. Don't get one of the ones w/ the little squares for stuff like xc because they are not nearly as protective even though they can be more form fitting. You get used to the more stiff ones after a few rides and don't even notice you're wearing it.
I've owned 3 vests, and by far my favorite is my current one, an Airowear Outlyne. They fit very well, are sort of a memory foam that isn't stiff once it warms up to your body temperature, and there are many, many sizing options, especially if you order it from England, rather than the US (cheaper too!)