Mike Nash announced Customer Preview Program on Windows Team Blog:
Beginning Thursday Dec. 4th, we will be making the Windows Vista and Windows Server 2008 Service Pack 2 Beta available to everyone through a Customer Preview Program (CPP). The CPP will launch on TechNet and be available to anyone interested in trying out this service pack. The CPP is intended for technology enthusiasts, developers, and IT Pros who would like to test Service Pack 2 in their environments and with their applications prior to final release. For most customers, our best advice would be to wait until the final release prior to installing this service pack.
So if you are brave and bold feel free to help Windows Team in testing of SP2 and make their WIndows products better.