From the SBS Blog:

We are looking for a few good testers to help influence the direction of this new Microsoft® Windows Server Solutions Product. Do you have three 64 bit machines that you can spare to test some of the most exciting technology Microsoft is working on? I know the system requirements are a daunting one, but this is the only way we can test "Centro". So, if you are already on the Longhorn Beta or the Exchange Beta, then you are already two thirds of the way there.

For more information, codes to access the beta, how to sign up etc, go look at The SBS and Centro Community Lead Blog : So, what is "Centro" you ask...........?