Windows Mobile 6 is confirmed to be released May 4th for the T-Mobile dash (the HTC Excalibur). But of course, you can download it here (queued download, I had to wait about 40 minutes)
Notably, WM6 comes with Windows Live built-in. The alarm also has a weekday / weekend option. Although not as nice as the multiple alarms of Pocket PC 5.0, it's definitely an improvement.
The upgrade process was very simple; as long as you have Active Sync installed, just hook up your phone and run the installer. It will take a few minutes and your phone will reboot a couple times. Make sure to backup any important data, as the installation will wipe all your old data off your phone.