"The primary driver behind expanding the reach of IE7 to Windows XP was security. That was the core of the message that we delivered last year at RSA and remains true to this day. The nature of the attacks you're seeing on the Internet, along with the nature of just how things have evolved and the worries that our customers have, made it a very easy decision to bring that technology to Windows XP for security reasons."
Another significant thing he mentions is that they plan to continue development of IE7 at the same pace, even after it has gone gold, to continue to improve and update it. Meaning, hopefully, that as problems are discovered, people no longer have to wait weeks or even months for a problem to be fixed. A revamping of how ActiveX is handled is also mentioned, a critical part of IE security. He also mentions committment to not dropping support for IE6, with security fixes still being provided for it as time goes on. If you are interested in the development of IE7 or what it will be like when it reaches you, check it out.