I updated the release script and download pages this afternoon. There are now two releases: stable and unstable.
1. The stable release is created from the last release of WordPress, and a "recent and tested" release of WPMU. They are WP 1.5.1.2 and rev 78 of WPMU from SVN.
2. The unstable release is like the "latest" tarball that was already there. It's composed of bleeding edge stuff, and *will* break throughout this month as we're going to update WPMU dramatically!
If in doubt about which file to download, go for the stable release, it'll cause you less headaches!