Update (21st October 2006):
Windows Vista Release Candidate 1 (RC1)—the final major pre-release of Windows Vista—is now available through the Customer Preview Program.
Windows Vista Customer Preview Program - http://www.microsoft.com/windowsvista/getready/preview.mspx
The recommended way to get a copy of Windows Vista at the moment is be an MSDN member, or to at least know someone who is an MSDN member.
If you are interested in becoming a beta tester for a program:
1. Sign in at http://beta.microsoft.com with your Microsoft .NET Passport account.
2. When prompted, enter BetaReq as the GuestID, and then go to the Potential Testers program.
Send chocolate to Microsoft.That's right, Adam, an eager Microsoft customer FedEx'd two pounds of inscribed chocolate to Microsoft and was accepted to test Windows Vista!
"May I please test Longhorn Beta 1?"