It would appear that Windows Vista 5308 is available for members of the public to download from Microsoft's servers. This is almost certainly a mistake on Microsoft's part, but nevertheless – the build is publicly downloadable and appears to be identical to the version available to testers via MS Connect.
Naturally, the build can only be installed by those with a valid Windows Vista Product key, which are issued only to official testers and MSDN/Technet Subscribers. The file is 2.71GB, and is named FebCTP_5308_32bit_Main_Staged_DVD.iso.
(Links Removed on request)