Today, Microsoft released its first wave of invitations for the Windows 7 beta test. The invitations were only sent to a small, select group of 'technical beta' testers.
In the email sent to invited testers, Microsoft states that the beta is not available for download right away. A reliable Russian website, Wzor, has claimed that the beta Microsoft will upload is build 7000.0.081212-1400, built December 12th. They also claim the build will be made available to testers within the next 48 hours.
In the email, Microsoft also mentioned their website where it has placed a wealth of information about Windows 7 and mentions the beta. This will likely be updated in the future. It should also be noted that Windows Server 2008 R2, included in this beta test, has a portal too. According to Microsoft's roadmap, this server-orientated Windows will have a longer release, scheduled to be released in 2010.
Neowin will provide updates on this beta when the information becomes available.