I've bought the Nanoloop1.3 for the Gameboy a while a go... I do some chiptunes while I'm on my bustravel to work.
Kinda funny, "mobile-generation"-kids starrin at u (haha a gameboy, how outdated, do you play tetris?)
Bleepy funky beats in a minute - thats pretty cool
But the "LSDJ (little sound dj" is more flexible and more used by chiptune musicians because its Trackerbased (like FastTracker, ModPlug etc) and it uses samples (4bit) but you can't by the cartridge on the net, only the software(ROM) is available for about 70€
and you need an extra Flashcard (+ ~50€) ... thats expensive ...
http://littlesounddj.com/lsd/ The cool thing is both (LSDJ and Nanoloop1.3) work on the GameboyAdvance.
great Gameboy artists are BubblyFish, Lo-Bat, 8cylinder, BitShifter, NullSleep, M-.-n ....
check out
http://www.8bitpeoples.com/