Magic Find does work on bosses, including Act Bosses. Whether it would or wouldn't changed several times during the beta, but in the final version of v1.10 it does work. Comments about this by code-digger Jarulf can be seen in this forum thread.
http://www.battle.net/forums/thread.aspx?FN=d2-general&T=483431&P=1&ReplyCount=21#post483431
