With the death of XP I would expect to see a usage drop, not only in IE6 but in IE7 and IE8 too.
IE8 is the latest version of IE that can be installed on XP, I would assume that most people using IE8 would be XP users.
If anyone has data on IE8 OS versions I would be interested.
For our app I've been monitoring IE8 usage for the last month. It is almost exactly one-for-one with XP users (YES!) and dropping a little at a time every day. When I first started looking the rolling 30 day average was roughly 15%. It has since dropped below 12%. I would imagine by the end of the year and a wonderful rash of XP viruses and worms, a lot of our customers will have had to drop both of them.
Except it is the browser that comes by default in Vista and Windows 7 installations at the corporate level, so XP has little influence on it.