Microsoft employee Nick White has posted some information on the Windows Vista team blog regarding the availability of RC1.
"Will Microsoft open testing sign-ups again when RC1 is released?
Yes.
Will the Customer Preview Program be available for those who missed out on Beta 2 to sign up and get RC1?
Yes.
If I participated in Beta 2 by downloading a PID and the build, will I be able to participate in RC1 automatically?
Yes."
Nick also hints that the latest build he has used is 5505. By Microsoft"s own admission, RC1 should be released at some stage soon when the quality and stability reaches RC level.