First Beta release of Windows 7 will probabilly available on Decembero 2008.
To have your copy of Windows 7 B1, you have sign up to Beta Tester program: go to connect.microsoft.com here you can sign in with your Microsoft Passport account and once you answer a few questions, you'll be entered the selection for the Windows 7 beta testing program.
According to Christina, the Windows 7 beta program will start very soon, but since pre-beta versions will be handed out at PDC2008 conference, it is likely that the beta version will follow a few weeks after that, possibly as late as mid-December. On connect.microsoft.com, the beta tester application form is typically made available weeks before the beta product is available for download; if Windows 7 beta 1 is released in December, Microsoft will likely be accepting applications as early as November 1st.
In the past, Microsoft has given priority to beta testers that have been testing other Microsoft products in the past as well, and provided useful feedback on them. If you have participated in other beta testing programs through connect.microsoft.com, it's recommended that you use the same account to apply for the Windows 7 beta program as well.