IE8 is at 33% share now. IE7 is about 7%. I expect most on IE6 are still on XP. So I'd expect that we'll see similar uptake rates for IE9, if not more given the HTML5 benefit (IE8 doesn't have much clear benefit over IE7 from what I can tell).

So I'd say expect IE9 to be closer to 27% in 3 years.

Of course this ignores two big questions:

1) Is IE still hemorraging market share? 2) Does IE9 actually reverse the trend of people using Chrome?