In Volume 145 of Nintendo Power magazine, there is a letter to NP talking about emulation (a follow-up to a question from Volume 143). The author basically says that copying a ROM that you own is legal, and brings up the "24 hours rule. Lastly, they also suggest that it's OK to copy games if they are not available in stores any longer (and are, therefore, public domain). Here's Nintendo's response, in it's entirety:

"We recieved a number of letters on this subject, so we'll take a moment to explain our position in detail. Having an emulated ROM of a game, whether you own it or not, is illegal. The 24-hour window is a common Internet rumor, but it's not true. The backup-copy theory, commonly used with computer software, does not apply to game data contained inside ROM semiconductor chips. You coulnd't make a 'backup' copy of the new Pokemon movie, or photocopy every single page of the new Harry Potter book, and the same logic applies to games. It's also against the law to download a ROM even if it's no longer widely available. Public domain refers to a work that has been around so long that it's free for everyone to use -- like a Bach composition or a Shakespeare play. But product copyrights are good for 75 years after they are files, which means it will be well past 2050 before any Nintendo games enter the public domain."

And to clear a couple of things up even further ... we consulted a copyright attorney on this matter, and his response was that Nintendo is right on the money. "Backups" were allowed by law in the day when software was being shipped on 5 1/4 and 3 1/2 inch disks, and were easily damagable. That is no longer needed with CD's and other durable media, and certainly not with semiconductor ROM chips. As well, the DMCA (Digital Millennium Copyright Act) pretty much negated all of these copyright "loopholes" anyway. None of the popular defenses (online "libraries", 24-hours, backups, etc.) would stand up in court were you to challenge them.

Lastly, he mentioned an important note. Just because a game is no longer available in stores, and the company that made the game may no longer be around, the copyright survives even the death of the company. The copyright always reverts to an individual or group in these cases. So don't think that just because a company is out of business that you couldn't be prosecuted.