CrossRoads:
Fill me in on what you changed. My IDE install at home is totally hosed from the mucking about I did to get the 644P working for the demo I wrote for you, can't compile anything anymore.
I want to remove it all, reload with -0021, and then update from there for the 644P again.
Er, sorry about that 
This is super easy though, although these cores are adapted for 0022. I doubt that will matter. If you do this and select the ATMega644P from the boards menu then it will use these, and probably use the 0021 cores for the native boards. This guy supports almost everything under creation. Not the next gen faster chips though. I was hoping for that! Atmel told me the newer ones have much better performance. Oh well.
Anyhow, go here:
http://www.avr-developers.com/corefiles/index.html
And like he says on that page, unzip his dirtro into the hardware folder. Thats it!