AVR-Chat

As far as I know any AVRISP MK2 (the USB version) will do PDI if loaded with the correct firmware.
The latest version of AVR-Studio - download from Atmel - will do this automatically when it detects an old version of software in the AVRISP.

Xmega is a very capable device with some nice features, but it has virtually nothing in common with the ordinary megas (apart from the assembler instruction set). All the hardware is entirely different- you can't "migrate" to it from ordinary megas, a complete rewrite would be needed.

-- 
Tim Mitchell