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?

I think there will be more bitcoin forks in the future like Namecoin or Ixcoin. The loyal miner will stay at bitcoin, but the greedy miner will try to exploit the difference in diffulties (and coins per block), assumed there is an exchange for trading in the other currencies into BTCs. Logically the price should be balanced at the ratio of the different difficulties (+blockcoins), if there is enough liquidity at the exchanges. I think we need some kind of chainhopper tools, or at least a page, where the current profitability for each pair are shown. I think this gets really interesting, if an IXC<=>NMC exchange starts, so there will be some triangle arbitrage opportunities... I hope http://dot-bit.org/tools/nextDifficulty.php will upgrade soon ...