Changing to 2.1 for pre-2.1 phones is actually quite a jump, with a lot of testing required. 2.1 to 2.2 is easy-peasy in comparison so there shouldn't be anywhere near as much for them to do.
Phones with HTC's Sense interface will need a bit of work before they ship the update, but no reason to think there will be the sort of delays we saw for 2.1...