http://www.guardian.co.uk/technology/blog/2009/may/01/windows7-rc-free

FTA:

Microsoft has announced that the Release Candidate version of Windows 7 will be available to the public as a free download on May 5. And since the RC version seems to be pretty much finished (I have not tried it), then it's essentially giving it away for more than a year: as mentioned earlier, it won't expire until 1 June next year.

Hmmm, learn from Vista they have (or so it seems)