Ultimately it's whatever is supported by browser makers. HTML 5 is feature rich, but the final draft may be years off. There are inherent difficulties in implementing things like audio and video support in 4(+) major rendering engines, and having them all behave the same way. Even validation would be a chore. Most browsers besides IE support the canvas element and SVG, but they still only represent about 25% of the market. With IE still commanding 75-80% of the market share, users who don't use or are oblivious to alternatives will be unable to use more advanced features, giving designers a tough decision.
IE8 is only finally implementing support which other browsers have had for users, meaning that the IE user base will always lag in compatibility. While HTML 5 is a nice idea, I think proprietary solutions such as Flash/AIR and Google Gears will continue to provide standardized support for the rich features HTML 5 provides. The biggest problem really is standardization - you have to design a website with the greatest percentage of users in mind as possible. There is hope, however. A Mozilla developer made a canvas plugin for IE - we could potentially see an open-source IE add-on that brings it up to a certain standard, that users could install much like Flash.
To Microsoft's credit they are being very open with IE8 and Windows 7 development (see their project blogs), so there is the possibility that more proactive IE development will accelerate adoption of HTML 5.