HTC had this to say according to EuroDroid:
You can now download the latest software update for new features and improved performance. It won’t delete any of your content and we’d recommend using a Wi-Fi connection as this is a large update. If not, standard data connection charges may apply. Any questions? Contact us via www.htc.com. After installing the update, you can confirm a successful update by confirming the Firmware version (From the Home Screen go to MENU> Settings> About Phone> Firmware version). The new Firmware version will be 2.2.
Update Information: Android 2.2 and HTC Sense update
Note: We still recommend you back up your all personal data first.
EuroDroid adds that the update hasn't yet hit all Desire handsets, and may not for some time:
As for the branded network versions, T-Mobile says it’ll arrive in “September”, Vodafone says it’ll hit its branded Desires “as soon as possible” and Orange, O2 and the others are staying rather quiet.
EuroDroid later reported that Orange expects the process to take about four weeks.
Check out the in-depth list of changes that come with Android 2.2 on XDA-Developers. Some of the reported changes include improved speed, more battery life, 720p recording, and a new Task Switcher, along with many many others.
Anyone that has received the update is encouraged to sound off in the forums and let us know what you like about the update.
Image from EuroDroid