Vista Build 5728 (which is of course after RC1) can be downloaded from here for a limited time. As per Microsoft, this build has improvements from RC1 but has not gone through the same regression testing as a RC or Beta, and it might not work in some situations.
Furthermore, Microsoft also highlights:
This build may not have the same level of support or servicing via Windows Update, and you may not be able to upgrade to the final version of Windows Vista.
(Thanks to fellow Avanut Ken Scahefer for pointing out the link for this one.)
Update: This build has some new backgrounds and a new start up animation (if that interests you). Matt has been running this build for a few days without any issues. Below is how the branching is working internally within Microsoft for Vista; you can read more about what this means here.
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