Thanks guys, I didn't realize I had selected a chip that's not really supported.
I'll try what you suggested. I have been using the Arduino 0022 environment with the 644P for a while now and this is the first time I had run into an issue. I'm not using hardware interupts myself, although I did have a plan to possibly use them. I can work around it though. If I do this right is there a chance that the SDFat library will work?