Maybe I'm being cynical, but something tells me that's right up there with New Year's Resolutions and the "I'M NOT GONNA BUY THIS GAME NOW" decree. We both know those things have lifetimes measured in hours, at best.
I think the simple economic reality is going to be that people are going to HAVE to go to the RMAH for loot they actually want. It's just too tempting to be able to make real cash on an item that is valuable, but not of particular use to you. Even if you sell it for gold, how much sense does it make to have billions of gold sitting around which you're never going to use? Sooner or later someone's going to see their huge pile of gold, and they're going to dump a lot of it for cash. All they've REALLY done is just sell that original item for cash which they THOUGHT they were selling for gold.
If you're right about the Diablo Arbitrage Moneymaking Metagame (and I believe you are), then the PRICE between comparable items will be about the same on either, but that's another issue - there are no such things as identical items in Diablo, even among legendaries. Those still have ranges of randomness to their stats.
So, a player is trying to see the difference between one on the RMAH, one of the gold AH, toss in some exchange rate math to see if there is any absolute price difference, and then divine what the difference in stats are worth. And, all the while, closing out one AH and opening the other back and forth, or at BEST page flipping between windows or tabs. Seems like a "compare" checkbox would be useful here.
Or, you could just see two similar items on one AH, not have to do any conversion, and look at the difference in prices in the same currency. You'd be able to do all of this on the same window, without having to bounce back and forth.
I just can't see a majority players who are playing an ACTION rpg sitting on the sidelines for even a spare minute to go through all that comparison mess. I think the RMAH is going to win out, particularly for hi-end loot.