Based heavily on "Handle MP3" by Shish.
FLV: Flash player
MP4: HTML5 with Flash fallback
OGV, WEBM: HTML5
MP4's flash fallback is forced with a bit of Javascript as some browsers won't fallback if they can't play H.264.
In the future, it may be necessary to change the user agent checks to reflect the current state of H.264 support.