There is the theory, that the price follows the difficulty. Let's now assume, merged mining is build into the current blockchains, lets say BTC, NMC and IXC. If the price is bound to the difficulty, the miner will mine an equivalent amount in every currency. Now with the hashrate being equal on all blockchains, the lower difficulties will raise and reach some level near below the top difficulty blockchain. If still the price follows difficulty, the exchange rate could reach nearly 1 in the long term.
I think, merged mining will come sooner or later, so the remaining question is: Will the price follow difficulty? And if yes, why?