How can I get the 2.2 version of Android on my HTC Desire and when?
I just got the update notification (unbranded Desire, Germany).
So the answer to "when" is: Now.
The initial leaked information was "late June", but this turned out to be incorrect. HTC have released a full statement now stating that the upgrade will be during Q3 (so anytime between now and the end of September). See this TechRadar article for more information.
Here is a good article about putting a custom 2.2 on your HTC Desire, I've heard first hand reports that it worked.