Maybe you can understand & help me go through Google Code Archive - Long-term storage for Google Code Project Hosting. where it talk about XCode so does this mean I could generate myself an Arduino IDE & slightly modifiy it, add a button to launch my own java application, add submenu to interact with my partner's Xmega board,... via java programming ?
Yep. I was successful in adding a button to the serial monitor to make the text being sent "sticky" and easy to send over and over, for example, without really knowing Java. The Arduino Source for Mac is already an XCode project, and compiled pretty cleanly last time I tried...
I'm not sure why you'd want to use Arduino instead of "your favorite editor" in conjunction with avr-gcc, avr-g++, and avrdude, which are the tools that Arduino IDE uses to do most of the work. (I suppose to make it easier for others to use it, eh?) You could take a look at the style and complexity of the modifications done for Sanguino as a starting point. The most likely roadblock is to find to you need a new version of avr-gcc/etc tools that have other problems withthe rest of the arduino environment.