The free Windows 7 Beta has gone live for the first 2.5 million downloaders. Keep in mind that the beta will expire on August 1st, so you will need to reinstall a commercial version. Updated 8:10 PM EST
Sure, 2.5 million seems like a lot, but those downloads will go quickly—so get on it ASAP (and expect the site to be overloaded). If you do miss out, no worries. You can still pick up the download on BitTorrent.
UPDATE 12:10 PST: Apparently there was some confusion regarding the start times. Word is that downloading would begin at 12 Noon PST / 3PM EST. That time has passed with no change—so you might want to keep checking back in.
UPDATE 2:00 PST: Lifehacker is reporting that the Microsoft 7 beta has been pulled due to to the unexpected strain on their servers. While they shore things up, there is no ETA on product keys.