I feel HTML5 today's "Web 2.0" of a few years ago. It's an overblown term that doesn't really capture what it is. CSS3 is cool. Canvas is cool. More native video is cool, I guess. The other new html elements are... ok... but quite frankily I'm fine with <div>s and changing them to <sections> seems annoying more so than helpful. HTML5 doesn't seem to bring anything radically new; most of what it allows could be done before, just with more workarounds and quirks. It's being hyped up as some next great thing, and not really a radical new development, it's much more incremental really.
And plenty of people are still using IE8 and Windows XP, and will for the next few years, so I don't think it's really more critical to learn HTML5 than over HTML4. Actually, it's hardly even possible to learn one without the other.