How does Atmel's XMEGA numbering scheme work?
ATxmega128A1U
       nnnLms


From the current parts list, I gather the first number nnn is the amount of flash in kilobytes. The letter L is one of A or D, the cheaper D parts have no DMA or DAC. The number m is 1, 3, 4 for 100, 64, 44 pin parts, and a U suffix signifies a part with USB.