the mp4 needs to be listed first to support some older iSomethings which wouldn't bother looking further down the list for what it needs if it wasn't #1. This was fixed in a later version, so now it's just to get so many devices as possible working.
*edit it was iPads with OSx3... there's a free upgrade to OSx4 which fixed this bug, but you never know who's upgraded.
Any browser that support ogg would support MP4 and/or WEBM, right?
Hm, no... FF3.5+ supports ogg but not WebM... FF4+ supports WebM. I'm still on 3.6x because Mozilla did a bonehead thing: they decided on a release-track of every 6 weeks (this alone's not so bad), but with immediate drop of support for the previous, meaning there'll never be another stable version of FF out there that supports my necessary-for-debugging-stuff plugins.
Some developers at Mozilla realise this and there's talk of having some stable long-term version while the 6-week-release thing continues on its own, like Ubuntu does, but there are also apparently die-hards who don't care if a browser actually works the way users want who are arguing against this. Prolly the same people who removed the Properties option from the context menu "because only nerds need it"... so someone made a plugin to bring it back. Sigh.
I wonder though if it's still recommended to list the codecs as well in the video code?
<source src="iStock_video.mp4" [i]type="video/mp4; codecs='avc1.42E01E, mp4a.40.2'"[/i]>
(notice the quotes around codecs, those are nested)
Are you also planning a Flash fallback?