MSDN Subscriber downloads has IE 7 beta ready for download! (It's under Operating Systems if you're having trouble locating it).
A comment from the IE team's blog
With Beta 1 done, you can expect much more active posting about technical details on the features that are now public. We’ve posted an IE7 Technology Overview, What’s New in Internet Explorer 7 for Developers, and an Anti-Phishing Whitepaper. Based on questions and feedback from the beta, we’ll blog and post more.