I talked to Atmel about the xmega parts a bit at ESC. According to the guy I talked to, their big feature is this "Event System" that enables the peripherals, in conjunction with the DMA controllers, to do a lot of "common" things without having to involve the CPU at all. Aside from allowing higher performance if the CPU is awake, this also allows the CPU to spend more time in assorted SLEEP modes, consuming less power.
I dunno if the features are that relevant to an arduino-like product. It's a form of multiprocessing, which is a whole huge can of worms from a software development perspective.