I think part of it is the move to devices. I have several sites I use that aren't business related that don't work with IE or warn me that there may be problems. I hit most of these on my tablet or phone.
Let's hope that IE 10 can win some of that back. In part that may have to do with the success of W8, WP8, Xbox, and to some degree SmartGlass. If Microsoft can get people interested in their ecosystem then IE's use may increase -- especially if SmartGlass takes off. If they were smart they'd do a SmartGlass business implementation for Windows that would allow you to extend your work onto devices like they show in their concept videos like this one. I bring up SmartGlass because it's HTML5/JS and am assuming that at least on Microsoft devices it will run [better] in the IE engine.