Avalon and Indigo Beta1 RC hits the download center
You can now pick up the Beta1 Pre-Release Candidate of Avalon and Indigo on the MS Download Center (RSS). This version was built using the Whidbey Beta 2 bits, so everything should be good to go to just dive right in! According to a blogger from MS these are the bits that will go live with Longhorn unless some really nastiness is found.
FYI: There is also another related download in case you want to try out the MSMQ queued channels with Indigo. They have a MSMQ 3.5 Beta available for Windows 2003 and XP.