Actually, (Speaking of ROMS), it is perfectly legal to use a Rom if you own the original cart. You are really only paying for the software. Emulators have a pretty easy time without getting sued or anything like that.
I would say, screw Nintendo and download all the NES and SNES games, except that they are rereleasing most of the popular titles for those consoles on the GBa, often with enhancements. So, I give Nintendo all my love.
As far as "abandonware" goes, as far as
www.the-underdogs.org is concerned, most of the companies who released those titles no longer exist. A lot of those titles were designated freeware by their original authors (As in games like 3 by Three and Fool's Errand). Plus, the way that copyright law is enforced in the US, the company has to claim suit against the infringing party. This can be either a cease-and-desist order or a suit for monetary damages.
I should point out, though, that in the case of some really good games that our corporate chain megastores will NOT carry, I would rather they be stolen and live on for future generations than merely fade away into so many dissociated electrons.