I think it's inevitable that someone will do it at some point. The key question for me (I need to study scripting capabilities in more detail to answer it for myself) is: if you have an alternate coinage (that is substantially like bitcoin), can you craft a pair of transactions in each block chain such that you have an irreversible exchange of one for the other. Some people have posted about utilizing transaction expiration features to implement escrow like and transactions that can clear immediately...I think something like that would have to be used. You would need to make sure both transactions on both block chains are embedded deep enough before the transactions are finalized.
The difference between being able to craft a pair of irreversible transactions on two different block chains and not is likely the difference between being able to implement a p2p exchange using bitcoin vs having to either use a new type of coin or make some pretty significant changes to bitcoin (if you want to make that p2p exchange be secure, irreversible, and only involve 2 parties). With some of the stuff I've read about using expirations and exchanging transactions privately before announcing them, I'm optimistic that it can be done, but as I said, I need to study the scripting capabilities in more depth.