I weigh ~250. No curb hopping or anything like that and when I do go over RR tracks or potholes I am out of the saddle. I will take a pic and attempt to post it from work tomorrow. Anyway, it is a 27.2 dia Bontrager Race black anadized aluminum seat post.

I'm thinking there was some sort of internal flaw and I hit the fatigue limit. When looking at the break, it looks to me like about a quarter of the circumfrance material is chalky looking and the remaining 3/4 looks like shiny metal... possibly it was just a poor casting.