I believe the way it works is that if you have purchased a game, you have the right to own a single backup in case the original purchased product becomes lost, stolen or unplayable (if the product is sold off or given away the backup becomes illegal and has to be deleted). It is still illegal however to posess two or more backups of the same game, as it indicates that it is being used for unlawful personal benefits.
This law also applies to Digital Media such as DVD's and CD's.
Don't ask me where the law is, but that's what I have heard.
That's what most believe the Fair Use laws are. I asked if anyone can quote the original law.
Under United States law, obtaining a dumped copy of the original machine's BIOS is legal under the ruling Lewis Galoob Toys, Inc. v. Nintendo of America, Inc., 964 F.2d 965 (9th Cir. 1992) as fair use as long as the user obtained a legally purchased copy of the machine. This has not included games, and I haven't been able to find a specific law or case that deals with it. It's a legal grey area.
By the way, the case I mentioned has to do with the Game Genie and has little to do with emulation, but it does talk about dumping game system bios files, and could technically be interpreted to refer to games themselves.
Here's the part that most forget/willfully ignore. Every time Nintendo, or any company really, re-release a game, they renew the copyright. While you may own that pristine copy of Master Chu and the Drunkard Hu (look it up) and dumped the rom yourself, the moment it hits the Virtual Console (and I'd shit if it ever did), that emulated copy is not legal anymore for that system. Fair Use doesn't cover that.
In fact, aside from making a new cartridge and playing it on the system it was made for (and you used to copy that wonderful piece of...ahem, history), it was never legal to play on anything else.
Fair use laws don't cover switching forms of media unless the original form is completely obsolete (like VHS or Betamax to DVD, and even then only home movies or items that never came out on DVD), and, if you dumped the game yourself, if obviously wasn't.
Now, admittedly, the case can be made that the NES is indeed obsolete, but you see the confusion Fair Use can cause.