Microsoft recently announced that they would push an update from IE6 to IE7 using the Automatic Updates feature in Windows XP. Users will have to allow the installation to take place, but I believe most of them will.
Why do I bring this up? It's going to result in much faster adoption of IE7 than most of us originally had anticipated. Considering that I've been expecting adoption to be quite rapid, that's saying something. This update has the potential to push many tens of millions of IE7 browsers into the market literally overnight.