  ::I'm an official tester so let me explain a bit. The preliminary release schedule for Windows Vista is that there are two betas follows with release candidate (RC) builds. Decision would be made in the development team to see which build/feature sets will be ready for the beta 2 build. Development still continues even the build has been decided, so the code will fork (or branch) for beta 2 release. When build 538x is in escrow for beta 2 release, the build number for the main tree is bumped up to 54xx and being considered as a post-beta 2 and pre-RC1 builds. The build number is just cosmetic so the beta 2 label persists even it is pre-RC1. 