After Dish Network, Newegg, and now Dell, I've come to the conclusion that merchant adoption in itself should have no long term effect on price.
I don't think there's been a case of merchant adoption having an effect on the immediate price.
Long term is where it's at. Every merchant adds legitimacy. More and more of them will follow Overstock's investigations into a closed economy.
The average Joe who vaguely knew of it as drug money a couple of years ago will look a little closer with each new merchant. Perhaps there'll be an option to be paid in BTC by quite a few of these companies too.
It's a process of seepage rather than an explosion. Eventually it'll pay off enormously. Without it, it's going to whimper rather than bang.