HTML5 is really starting to make some ground. With YouTube successfully using HTML5 for video, and iOS4 not supporting flash. If you want to be displaying video on an iphone, then you are going to want to use html5.
After diving in head first, I am a little worried about the non strict nature of html5, having used xhtml for a while, I really like the xml style of html writing and will continue to use this code style, because for me it is much more readable.
