Interesting. YouTube must be a mess with all of these competing formats it needs to support. I assume it will eventually switch to WebM for both HTML5 and Flash by default and just use h264 for compatibility. Still, it must be horrific.
Nope. It was stored as h264 before they released webm (and then flash played it). When they released Webm, they started to convert all that stuff to VP8 (my guess is they should be through by now), so it's only VP8 now (via HTML5 or flash). End of story.
Adobe has not yet shipped a version of Flash which supports VP8, or WebM for that matter. They've remained silent on that matter since their initial announcement at Google IO last year.
This is incorrect. They are still retaining and serving h.264 to mobile devices, since WebM has zero mobile hardware support.