There were some reports about the nature of Francisco Liriano's mysterious non-pitching-arm yesterday, but I waited on mentioning them here until we had something more reliable. His broken bone is his humerus, which is basically his upper-arm bone. We still don't know how serious it is, and I'm guessing we probably won't until Spring Training.
It's still unclear how he broke the bone, but yesterday, Tom Singer found a Spanish-language article reporting that Liriano broke it during a fall in his bathroom at home.
Anyway, we still don't know exactly what happened. I suppose we do, however, know the result, and at least now we all get to make not-very-humerus jokes about it.