Some users of the HTC Desire began receiving the Froyo (Android 2.2) update at the beginning of August, but not all European carriers were ready to roll it out just yet. One of these was T-Mobile. But the update is on its way, starting today.
From the support forums of T-Mobile's UK site:
We have some exciting news for all our Desire users.
The 2.2 firmware update (FroYo) will be available from today and will be sent over the air (OTA) direct to your phone over the coming weeks.
Be sure to save all your personal data before you start downloading.