The battle between Apple and Adobe over Flash, which has been joined to a lesser extent by Microsoft and Opera, is almost certainly going to end with HTML5 as the dominant way of delivering video and interactive content on the web, with Flash becoming the RealMedia of a new generation. It'll be used by a few people, especially in legacy applications, but will fall out of mainstream use. However, that's not where we are now.
Today, Flash is still very much a mainstream technology, HTML5-compliant browsers implement different portions of the standard, and HTML5 itself continues to evolve. There are no graphic authoring tools capable of creating anything coming close to Flash-based interactive content in HTML5. Yesterday, Scribd announced that it's starting to shift from Flash to HTML5 for its online document sharing platform, after a six-month development effort. Moving from Flash to HTML5 for video is now fairly easy, but for interactive content, a major development effort is usually required.
So we're still far away from the time that HTML5 will be ready to take over for all of Flash's applications, and the decision tree is now very clear: If you need to access Flash content or applications, forget about using any device that runs on the iPhone operating system. Once Adobe releases Flash 10.1 for Android, that won't be such a big obstacle. If you're developing applications that need to live for more than a year or two, however, you should seriously consider wading into the HTML5 pond now to avoid having to rebuild all your code for HTML5 later on.