To continue on Greg's comments.
H.264 .mp4 can work in:
Flash
Quicktime 7
Silverlight
Windows Media Player 12
HTML5 (current Mozilla browsers the exception and that's about to change) See this post
Codec War Over - Firefox to support H.264
So that pretty much covers everything.
Basically Flash with HTML5 fallback or vice versa would cover everything and you can use the same file in both.