I have certainly more than dabled in Gameboy (Classic and Color) games hacking and producing, but only for fun. Unfortunately, since the days of the NES I've been negotiating deals to make Nintendo cartridge games, and its A: Very expensive and B: Very difficult to get approval. Basically only Nintendo are allowed to release or at least endorse carts, so you buy a dev kit for about 10 grand, then spend years making a game, only for Nintendo to say "Oh Nice game but we wanna release a similar product, so sorry you have to scrap it"

Grrr The Nokia N-Gage seems a better option for the future, Raven and I and a few others are looking into that.
[Edit] I should have mentionned that you can buy blank cartridges for "backup" but I've only ever seen them for quite high prices, and Nintendo would sue your ass off if you tried to comercially release anything. Strangely tho I know of one company who made GameBoyColor games recently and their games were for sale in a few shops for a while, being marketted by Datel. Plus a double game cartridge was given away free on an unnofficial GB magazine last year. It was kinda cool, I've got it somewhere "Space Invasion / Karate Joe" I think its called...