Hi Hilljockey,
I hope you didn't come off the bike when it went.
I had a similar thing happen to my left crank about 6 months ago. I wash the bike every week and didn't notice anything out of the ordinary the day before it went. I had just finished an interval session on the road when I felt something knocking on the back of the bike and the pedaling action felt a bit weird. On inspection it was a horizontal crack in the middle of the crank. So I nursed it to my bike shop. My mechanic here is an ex Team CSC mechanic and he said he's never seen it happen before.
It was an FSA gossamar crank on my Felt road bike. I didn't look into it in terms of warrenty/defect because I got it off Stoj 3 yrs ago and now live OS. So I just put a unlegra crankset on it.