Bruce Lawson of Opera gave an interesting presentation on this at OSCON, see his slides; the verbal conclusion was that it's OK, though there was some debate. Of course that's from somebody working for a browser maker so they're going to be less worried about older browsers (since everybody upgrading is to their advantage;-). The more you need to worry about older browsers, the worse your life is in general of course, but html 5 wouldn't help you there, sigh!-(
