I have never seen a legendary from a boss drop. From my understanding of the way loot works, it should be possible though. Everything in the same act (save pony land) has the same loot table. Just compared to elites and champions we kill bosses a hell of alot less. Would make sense for some of the legendaries like the Butcher's sycthe to actually come from the Butcher though.
I don't know what you all are complaining about. I've never found a single legendary item period. Not from a boss or elite pack or anyone for that matter. How do you get such good gear to get your magic find percentage so high?