How legal is this?

ROM hacking, on the other hand, is perfectly legal. Nintendo couldn't care less about hacks...unless you're selling them. For perfect legality, your rom must come from your own game cartridge.[/QUOTE]

But what if you make the ROM, and someone else sells it under your name?

It is semi-legal. If your in the U.S.A. (like me) it is legal (in most states) to own a game and back it up to a rom. I don't think it's allowed in the U.K. and Australia, but no one really cares enough to come after you, unless you try to sell it. The one exception of this is chrono trigger. I have personally seen at least seven different rom hacks get letters of cease and desist from square, not to mention that one 3d fan game... To answer your question, it is (usually) more legal than pirating movies, but less legal than backing up a rom.

It's a grey area but I'm pretty sure that Nintendo doesn't care anyways. It should be alright just as long as you don't go around selling bootleg carts or trading the roms themselves. Distribute your hack as a patch and you should be safe as long as you stay very far away from a company called Square Enix.

Nintendo doesn't really care too much about romhacks themselves. Especially when we're talking about GBA games and lower, which aren't sold anymore. I think they realize that there is a pretty good size community out there in general. Just make sure that you're distributing an IPS patch or something instead of a rom itself. If you want to be extra sure, your base rom could be dumped by yourself. And no selling, no matter how tempting it might be to give people the "original feel." If they want it that bad, they can get a flashcart to play their games with. It's worth keeping in mind that emulators themselves are legal, while the ROMs themselves may not be.

However... I should warn that there are companies who do care. There was once a Chrono Trigger sequel hack named Crimson Echoes that got a C&D from Square for example.

^^^Basically what Projectwolfe said.

More over, I want to make Downloading a rom more clear.

Downloading a ROM is illegal, however the action is justified only if you are doing so to treat it as a back-up. You can review every law, and look into any or all loopholes, its there.

...The only other time a ROM becomes LEGAL to download is if the country it originates from decides to NOT release to your country! Magi Nation for GBA for example. They made a japanese version of an American card game. They released it to their public and not us! THE GAME EVEN HAS A FULLY TRANSLATED ENGLISH DIALOG!

Honestly when it comes down to Pokemon rom hacking we are a niche. There aren't very many of us and those that do it often attempt to make the games better. If no other reason exists for Nintendo to allow us to hack the roms it would probably be that they want to see what we want out of the games. Its not as if we are cracking open Final Fantasy XIII-2 and hacking that. Look at games for PC such as Left 4 Dead. You can mod the crap out of that, do what ever you want just as long as your hack doesn't effect multi-player. But unfortunately you cannot do this on 360. It is all up to the system's owner. Nintendo is having a hard enough time financially hanging in. It would be unwise for multiple reasons to even draw up a post-it about rom hacking.

Oh, you hear that bro? It's the feds! Ha ha just kidding. No, its illegal, but as long as you don't sell it nobody cares.

I would say it's not illegal, provided that you don't sell the hack or provide a rom of it. Instead, make an ips of it.