Hello Sj,
An old theme just came up on the Wikipedia mailing list again : Wikipedia strongly encourages[1] using theora encoding, strongly enough that I don't think we currently host any other video encodings...[2] however, most users know how to make and edit and play mpegs, and find theora hard to work with.
As soon as a content author has got an avi/mpeg/something all she need is to recode to theora, which is not that hard.
On Linux/FreeBSD I am using the theora-encoder script, directly runnable from Konvalo.org (given a Coda client, available there as well).
For Windows, I think http://www.illiminable.com/ogg/ contains the necessary information.
http://mail.wikimedia.org/pipermail/wikipedia-l/2005-March/038244.html
Perhaps the theora-dev list can help us overcome our technical problems so that we have a reasonable choice in setting policy...
The problem of teaching the browsers to launch a right player is a bit of out of our control. A wikipedia page telling how to set suitable options for different browsers possibly would help.
www.theora.org contains links to sites hosting downloadable binaries which can play theora. Besides, mplayer from Konvalo.org can do it (on Linux and *BSD) as well.
Best regards, -- Ivan