WMExperts have are reporting here that the Official Windows Mobile 6.1 upgrade is available for download at HTC, here.
HTC lists the benefits of the ROM thusly:
- Upgrades the operating system from Windows Mobile® 6.0 Professional to Windows Mobile® 6.1 Professional
- EV-DO Rev. A functionality for faster download and upload speeds
when utilizing data services wherever EV-DO Rev. A coverage is
available.
- Advanced GPS functionality including the Sprint Navigation application.
- Adds the Opera browser.
- Adds a full Qwerty on-screen key Qwerty keyboard.
4. Adds the Opera browser.
I'm guessing Opera Mobile 9.5 like the HTC Diamond.