If Microsoft gets its act together and makes a browser that people don't mind using (and I believe IE8 is significant progress), I could see that level off around the 50% mark.
Similarly, if Webkit or FF makes some tremendous progress or has some killer feature, I could see IE's share drop very abruptly. So far however, the competition has mostly gone for feature parity.
One of the things that bugs me about the IE6 browser wars is that the majority of outspoken people think supporting standards would help. In nearly all fields (with healthy competition) supporting standards is something you need to do before anyone will adopt your product, and then you get to worry about new features.
I think this mindset of standards compliance has caused FF and Webkit to not push more interesting ideas. Want to support a video tag? Do it. Just make sure you follow the HTML 5 spec first. That's where real innovation comes from -- for instance, the creation of AJAX wasn't a standard, but Microsoft adopted it, and the other browsers thought it was a good idea.