Now that Windows Mobile 6 is announced, there is also a good article on MSDN that walks through the new developer features in WM6. The article is here, but this is my collection of notable features:

• Compact Framework 2.0 SP1 and SQL Server Compact Edition are in the ROM of new WM6 devices
• Device Emulator 2.0 (incl power state, phone profile, cellular emulation)
• Fake GPS, allowing to test GPS functionality
• Basic support for JavaScript in IE Mobile allowing basic AJAX functionality

Oh and yes, also the naming of the device families have been rehashed:

Windows Mobile 5.0 for Smartphone becomes Windows Mobile 6 Standard
Windows Mobile 5.0 for Pocket PC Phone Edition becomes Windows Mobile 6 Professional
Windows Mobile 5.0 for Pocket PC becomes Windows Mobile 6 Classic