Microsoft released Windows Vista Service Pack 1 Release Candidate 1 (Build 6001.18000) yesterday (01/24) for beta testers.
The last public release of Vista Service Pack 1 RC build 17128 is still available for download at Microsoft .
Build 18000 is referred to as Vista SP1 RC1 (KB937287) in Windows Update and not to confuse anyone is also referred to as Windows Vista SP1 RC Refresh 2. It is around 534.4 MB to download and install – very slowly one might add
Update: Microsoft has confirmed that this build is identical to RTM bits. So if you have this installed, you do not need to uninstall and reinstall Service Pack 1 RTM. You already have it.