I'm on a project where size and weight is more an issue than effort and price.
I'm asking you, because I want to be sure it works:
I can burn an atmega2560 with the Arduino Bootloader and then I'm able to work with it like with any Arduino? Same IDE, same coding language, same libraries and so on. Right?
Yes you can. The atmega2560 is "officially" supported in the arduino IDE =P! Just burn the bootloader that comes with Arduino IDE 1.0.1 and you're good to go! To me the atmega2560 is just a pain in the *** to solder =P!! May I ask what you're building/making?? If it's weight you're concerned about then I guess it's something that's going to fly eventually =P??