It's a popular handset so custom Rom of the next version is a given.

Officially, no one knows

 

I do think that Android 2.3 will come to Desire.
3.0 I'm not so sure about, that depends on whether it's in 6 or 12 months (by then the Desire will be labelled as 'old', disagree as I might).

That's the official story.

But of course the Desire will run both 2.3 and 3.0 even before it's out officially.