This is already happening. Litecoin is now trading at about $4 per LTC. It is slightly different in that average block generation occurs every 2.5 minutes inst

This is already happening. Litecoin is now trading at about $4 per LTC. It is slightly different in that average block generation occurs every 2.5 minutes instead of 10 and the limit is 84M coins, but otherwise it's basically the same idea as Bitcoin. There are numerous others -- TRC, PPC, NMC, DVC, etc -- that all are slightly different from bitcoin and are actively trading, although their exchange rates are much lower and extremely volatile. Namecoin is actually pretty interesting (embedding a domain service in the protocol), but the others are mostly copycats. http://dustcoin.com/mining http://dustcoin.com/mining has the most extensive list I know.