yeah - or nay?
Potentially this could be great; potentially it could also be the most damaging thing to happen to web development since the IE/NS wars.
Currently it's quite possible to write cross-platform, cross-browser compatible JavaScript that works with all the "main" browsers (Firefox, Opera, Konqueror/Safari) - even using AJaX. It's just IE that doesn't play nicely with others (ever); the DOM is broken, the CSS implementation is broken, AJaX (on IE) relies on an activeX control (which has changed since IE6) and is accessed in a slightly different way to how almost everyone else does it. Incidentally ICEBrowser works differently as well so it's not just MS, but IE has the biggest market share so it's far more significant when it doesn't work.
This will be GREAT if MS actually throw some money/time at their aging hunk-of-crap browser and bring it into line with what everyone else is doing. Once IE had market dominance MS just left it to rot (as it's not a money-spinner) - it's so far behind Opera in basic functionality it's unreal... but since it's all tied into mshtml.dll I suspect they're stuck with at least some of their own stale faeces as I doubt anyone actually knows what will break if they alter that dll too much (there's a lot of third party apps that sit on top of it). They might be better starting IE again and doing it right... but that of course would separate it from the core of the OS and remove their ability to bundle it with Windows (without running into yet another abuse of monopoly position investigation).
I'm all for MS/IE adopting the standards that are in place (the css box model would be a good place to start), however judging by past practices it seems more likely they'll try and twist the standards to suit themselves... and if they manage that, everything that ever worked across all the other browsers/platforms will become obsolete.