Just to give a bit of history. I went to unofficial Froyo 2.2 then flashed back to 2.1 a few weeks ago.
I just read the news about the official release. It said that you could just go to settings / about phone / updates but when I tried that it didn't work at all and said my phone was up to date.
After much fooling around, I figured out the problem.
I still had update.zip in the root of my phone that was left there from the flash to unofficial 2.2 - I deleted this file, went back to settings / about phone / updates and immediately I could update the phone.
Just thought I'd post this problem / solution before someone else had the problem.