Mozilla is retards for supporting OGG for the <video> tag, which to be totally honest.. Who the hell uses OGG for anything?! (No offence, Scawen).
Apple won't implement OGG as they don't want to get tied up in a potential patent case (as Ogg/Theora hasn't been proven to be legally patent-proof), so instead support something that people use, and that they have the rights to; H.264.
IE is a moot point, as they've always been the special kids that while everyone else is in class, learning maths, they're out painting tree's and chasing butterflies.
the Video tag is a trainwreck, Google and Apple are going for the H.264 way, and Opera and Mozilla are going for Ogg/Theora. While Microsoft are trying to push Silverlight on everyone as their alternative to flash+svg+<video>+etcera. Flash is tragic as it's slow as shit, Silverlight/Moonlight (Linux) isn't any better than Flash, and everything being in HTML is "good", but will never be successful as everyone has their own idea of politics and implementations of a layout engine.
Opera has their own engine that has quirks, Mozilla has Gecko which has quirks and isn't great, IE/Trident is a ****ing trainwreck, Safari/Chrome/Webkit has quirks, but it's been proven to be more accurate with web layouts, even with sub-standard HTML and CSS.
Internet browsers suck for this reason, however there is still hope, as unknowingly, more and more people are using Webkit based browsers (Chrome, Safari, MobileSafari, Future Blackberry Browser, etc).