Thanks everyone for the speculation and tales of commiseration. To add a little clarification, I've only every had two wheel sets on this bike, the one I bought it with (5000mi ago) and the one I bought off of ebay to replace the first failure. I did not build these wheelsets, but the spokes look to have only every been laced once--as far as I know. I'm going to chalk this up as 'one of those things' and 'just dumb luck'.

Do I need to de-lace the broken hubs to save the rims?