Alas, I hear that the xmegannAm chips are buggy. The nominal replacement is the xmegannAmU (with USB, even if you don't plan to use USB) chips, but they are not very available 
More info on avrfreaks...
Putting Arduino on it shouldn't be a big problem. Working with (new) peripheral structure might be. It's easy to end up like some of the non-AVR arduino attempts - having a lot working, but not enough to run the various obscure libraries that people want to run...