Wahoo! The public beta of SBS2008 is available as of today (5/28/2008) from the Microsoft download site. If you interested in testing this release candidate, registration is required. Hardware requirements include a 64 bit platform, 4gb memory, and a 60gb hard drive.
Remember … this is beta software, and is not to be run in a production environment. The purpose of the beta is to get more hands and eyes on the software and to provide feedback to Microsoft.