I feel like only a bold move like that would give IE any hope of preventing a further slide into irrelevance.
So if they were to implement HTML5 as it stands today, they'd release IE9 to find that the other browsers had coordinated with the W3 to release the final standards alongside FF4 and a new version of Chrome.
That's how it worked back when NN6 came out, and how it's worked ever since. It's hard to have sympathy for Microsoft in particular in this area, but that's what they're up against.
An example of this would be the CSS Box Model, which had reference implementations in both IE and NN at the time the spec was written. The spec changed how width was calculated, leaving every existing browser incompatible, and leading to silly things like Netscape.com not rendering correctly in the first released version of Netscape 6.
Nowadays, it's a mystery why MS gets left out of the loop. The standards committees are still non-MS dominated, so conspiricy-theorists could conclude that there's still some collusion between the Mozilla & WebKit teams and the W3. Or it could simply be Microsoft not wanting to get burned yet again.
There's a reason why Twiddla is so fast on IE. It's because we intentionally leave the Doctype of the whiteboard, thus enabling IE to use its fast Quirksmode rendering engine. Testing showed that adding a Doctype didn't effect FF or Chrome performance at all, but simply slowed IE down to their level.
IME, the current trend is that there are still quite a few people using IE6 (the hold-out corporations) but they are dying out. However, a lot of other IE users, perhaps even most of them, do upgrade reasonably quickly now, because there isn't the same incentive to stick with something like IE7. Since users of other browsers also tend to upgrade fairly quickly because everyone has auto-updates these days, I am optimistic that even relatively simple steps like including some CSS3 and SVG in IE9 will make a big and fairly rapid difference in practice to what a web designer can use in production sites.
I guess I was more looking at it from Microsoft's perspective. Are they serious about competing? If so, merely matching the features that other mainstream browsers have had for some time now isn't going to get them very far. Keep in mind that by the time IE9 is actually released, we'll probably have already seen major version releases from the other players.
To me, full CSS3 support and at least an attempt at full HTML5 would be a bold way for IE to jump back into the race as a true leader, not just an also-ran contender.
As one anecdotal contribution, I now know more people who have switched back to IE recently than switched away from it, and these are all technically knowledgeable people who are well aware of the alternatives. They're just fed up of the numerous flaws in the rival browsers and feel they get a better experience with IE8. Looking objectively, it's not hard to see why.
Firefox now runs slower than a sleeping tortoise and claims more system resources than Windows Vista on a bad day. Its main strength has always been its plug-in system, but that same system also causes both stability problems and multiple obvious but apparently WONTFIX security and privacy risks. It still runs all tabs under one process, so it is laughably easy to lock up the entire browser and lose anything you were doing on other pages.
Chrome has some basic rendering bugs, ironically including CSS3-style rounded corners, which have actually been getting worse in recent versions. And it, too, has its share of privacy concerns for some people, not the least of which is the name "Google" that appears before the "Chrome" part.
I don't run Safari myself, but I hear cursing regularly from colleagues who do about pages that simply don't work, so clearly all is not well there either.
In comparison, while IE has moved slower than the rest of the browsers, IE8 is already reasonably quick, renders the pages non-geeks read reliably, actually has better tab isolation than anyone else, etc. If IE9 does bring the same kind of performance jump for JS execution and support for the most useful CSS3 features, so it catches up with the recent trends (which, in fairness, only really took off after IE8 was out) then I'll be tempted to switch back myself.