A lot of people have been very cautious about using HTML5 in large part because of IE's relatively poor support for it. While IE9 has been an improvement, it still does not support HTML5 as well as Firefox or Chrome. The upcoming IE10 looks to change that and add support for many of the more cutting edge HTML5 features. The fly in the ointment is Windows XP users who are still stuck with IE8. As a result, while I am optimistic about IE10 helping the HTML5 cause, I suspect that the "pry XP from my cold, dead fingers" folks are going to be the reason why HTML5 won't be universally usable for a long time.
Justin James is the Lead Architect for Conigent.