HTC has released its first two Windows Mobile 6.5 upgrades. The updates are available for the European versions of the HTC Touch Pro2 and the HTC Snap. I expect that upgrades for other models (like the Diamond2) will be announced shortly.
Carriers have also started to release upgrades. T-Mobile Germany has released new ROMs for the MDA Compact V (its variant of the HTC Diamond2) and the MDA Vario V (its variant of the HTC Touch Pro2).
Note: The update will replace the current ROM image and will erase partnership, all user data, and programs on your device. Be sure to back everything up before you start.