We're pretty much done with the features for 1.5 at this point. Other than a non-functional recent item list for projects in the GUI, we believe we are feature complete. We will probably be restructuring some of the GUI runner code so that it can have a long and maintainable life. :)
The big new feature for 1.5 Beta is the introduction of xUnit.net Test Project files (.xunit files). These files contain information about multiple assemblies and their run parameters, and can be consumed from the GUI and MSBuild runners (as well as created in the GUI). To accommodate this, the GUI got a small makeover in the menus, and now behaves like a single document application (where the document is your project file). We also support launching .xunit files from Windows Explorer (run the installer utility, xunit.installer.exe, to enable this support).
The other big feature is that we've included builds of the console and GUI runner which always run in 32-bit mode, even on 64-bit OSes. The default versions will run in whatever is native. We also included environment information (32- vs. 64-bit and the .NET version) in several places, including the native XML output files.
Finally, we added preliminary support for ASP.NET MVC 2 Preview 1. This support may need to evolve with later releases of MVC.
As always, we look forward to usage reports, especially with the new GUI. Your help lets us know that we're ready to ship!