The great folks at Atmel sent me some sample 644s, but I have no means of programming them. I've tried using two Arduinos, Uno and Mega, as ISPs and neither one can burn the Sanguino (or any other) bootloader, through AVRdude or Arduino IDE. I have no money for a purpose built ISP, sadly, and I want to be able to upload sketches through Arduino IDE one way or another.
Try Nick Gammon's bootload installer. 644 should use same bootloader as 1284 (same chip, twice the memory).
I just couldn't figure it out, that and I don't have a programming cable. Any other ideas?