I mean that you might run into a situation where "atXMega support was added to gcc version 4.5, but gcc versions higher than 4.3 have a bug in the C++ compiler that prevents the Arduino xxx feature from working." For example, the 4.3.2 compiler currently used in arduino seems to have a C++ specific bug in PROGMEM datatypes. In that case, there is an easy workaround and PROGMEM isn't widely used anyway, but it's not hard to imagine a more serious issues (there WAS one that cause some version scrambling; I don't remember the details. I don't think you can currently use gcc 4.4.x with arduino...)
Do you then recommend I stick for the moment with my mega board + arduino IDE and wait a while before engaging into ATXmega board & associated SDK ?
P.S. Merry Christmas 