By now, you've probably heard that database mirroring and the SQL Express management interface will not make it in the November release of SQL Server 2005. (You can read Paul Flessner's public letter about the decision at http://lists.sqlmag.com/t?ctl=145CF:7B5CC.)
Although the feature has slipped, it is still available as an undocumented feature through the application of a trace flag. And since the feature is still beta, Microsoft wants to hear about any bugs or improvements you might suggest.
In fact, even though Microsoft isn't officially supporting database mirroring in this release, I encourage you to go ahead and use it (in a non-production environment of course) anyway. Note that you can also get support for your use of the feature if you're a member of the Microsoft GoLive program for SQL Server, which lets early adopters go into production with pre-release software. One of the main reasons Microsoft delayed the release of database mirror was because they needed more beta feedback. I say "give it to 'em!".
Finally, you can find the full list of September CTP features and order the SQL Server Readiness Kit CD at http://lists.sqlmag.com/t?ctl=145D2:7B5CC .