We have yet another mailing list. In order to move discussion of nightly builds off of the support forums, a testers list is now available. If you use nightly builds or any pre-release version of WordPress, please join the list. Hopefully the list will allow testers and developers to communicate more effectively while relieving the support forums of nightly build related queries. The support forums need to focus on supporting stable releases.
The current nightly builds are 1.5.1 alpha builds. 1.5.1 is a bug fix release for 1.5. If you would like to help with 1.5.1 testing, please join the testers list and try out a nightly build.