TechNet

If you have a TechNet Plus account, you'll already get the beta content, but if you want to test drive these new programs, especially Exchange 12, you can sign up at the TechNet Pre-Beta Release site.

From the site:

Lead evaluations for new technologies and understand their impact on your IT environment and their potential for your company by pre-registering here. You will be the first to be notified when the Windows Vista, Office "12," and Exchange "12" public beta releases become available. Additionally, you will receive updates and information regularly through the Microsoft TechNet Flash newsletter, from the time you pre-register until the launch of the product.

Link:
http://www.microsoft.com/technet/prodtechnol/beta/preregister.mspx

Looks like it will be mainly covering wireless security and Windows Defender aka what used to be Microsoft Anti-Spyware, which used to be Giant AntiSpyware. It has come a long way since then though, especially for Terminal Servers. Giant used to launch 3 processes per user session, ouch.

Register for it here:
http://msevents.microsoft.com/cui/EventDetail.aspx?culture=en-US&EventID=1032291929

aka

http://tinyurl.com/n6ydd