The HTML5 trial has always been opt-in, but also features H.264 files. WebM has been the default for HTML5 since they introduced it, only falling back to H.264 if WebM is not available (or back to Flash if the video require adverts).

I think the big change is that they're now converting all uploads to WebM, previously they had to be above 720p OR uploaded in WebM to trigger this. Presumably because they've converted enough of the popular "classic" videos that they can now spare the encode time for all new files. Just another milestone on the journey towards having everything in WebM.