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 official Froyo release for the HTC Desire seems to be imminent. Though since it's already been over three weeks since they said it would come within the next two, you might have better luck with an unofficial ROM.
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.