During the weekend Microsoft released a new build of Windows Vista to the public. While initially only available to registered testers, the new post-RC1 build
can now be downloaded by the general public for a limited time. Although no specifics or release notes have been made available, you should expect a further polished build than RC1.
This build (5728) has a number of improvements and updates from RC1, but has not been put through the same internal testing process as RC1 and therefore may be unstable in certain installations. We are making this release available for a limited time only (and only by download) in order to get broad distribution and testing in a variety of PC configurations.
For those already running RC1 comfortably I wouldn't recommend upgrading until a more mature build is released. For the adventurous ones (like me I guess), you can expect fixes here and there plus some speed improvements, but like Microsoft said, this might not be the case for everyone.