Microsoft released Vista RC1 (build 5600) to users who participated in the Customer Preview Program today. If you participated in the Beta 2 release, you should have received an e-mail with information on the RC1 release. You will need to use the key given to you in Vista Beta 2 for this new release
If you were not part of the Customer Preview Program before don't worry. Microsoft will be taking new participants soon, so check the CPP website for information on signing up.
Vista Customer Preview ProgramDownload Vista RC1