@adrian: this sounds awesome and definitely fits some of our needs just fine.. so thank you very much for working on this!
Regarding ffmpeg and ffmpeg-php, I've just tested two hosts and one had both of those installed. Other host is my Linode VPS which, as expected, didn't (and shouldn't) have them yet.
I don't think this kind of requirement is such a big problem really, especially since this kind of field can IMHO be most useful in slightly larger projects, where getting a proper host shouldn't be an issue. YouTube is just fine for most others.
Have you seen https://github.com/CodeScaleInc/ffmpeg-php, by the way? I can't speak for it's quality as I've never tried it, but it might be worth considering as an option to ffmpeg-php. Probably not a good default though, as a PHP extension would most likely have some advantages, such as speed (C vs. PHP.)
From your feature list "automated video format conversion" sounds like the most important one to me. Unless I'm mistaken, video element only takes us halfway there; visitor's browser still needs to support each video format used and thus conversion + some kind of player sounds like the best option. One system I've been working with has something similar as this, though it by default converts all other formats to FLV and then uses Flowplayer to play those -- this has been in use for years and seems like a solid option.
Anyway, I'll be sure to check your code better as soon as I can. Would love to help somehow, too.
Edit: almost forgot to add that having an optional, built-in player would be a nice addition. Something like the Flowplayer integration I mentioned above, so that $page->video_field->render() would simply output the player. Just saying