I believe there's a good possibility the D3 RMAH won't be the last RMAH from Blizzard we see (assuming it goes well, of course, which I believe it will be fine).
disclaimer: I don't play WoW
Recently, Blizzard has been selling a mount in WoW that can be traded on the Auction House. This is a big change since every previous mount was tied permanently to the owning account. Anyway, this means one could arrive at a dollars per WoW gold figure since real money can be "injected" into WoW through the mounts. Essentially, they're selling gold! So, Blizzard feels safe with WoW gold as a real-money analogue, to some extent.
Also, WoW had the auction house in it from the beginning. I'd imagine running such a beast is a lot like running a free eBay. There are servers, server maintenance, potential transaction issues, bandwidth consumption, and not to mention the development time. I think they felt it was a good feature, and it is, but they might have overlooked the maintenance cost a bit. Surely they've nailed it down to a science so its smooth to operate now. It costs money to operate an auction house, even if its free!
Regardless, once real money is introduced, the system must be essentially bug free, and they must to succumb to "the customer is always right", potentially handling slews of customer issues (i.e. I bought the wrong item; I want a refund; Where's my item? ... ad infinitum).
Diablo 3 will no doubt be a big game in terms of sales figures and concurrent users, however its safe to say it won't top WoW. So, the system must be thoroughly tested to handle such a massive audience.
So, to me, Diablo 3 would be a great test bed for a new profit center for Project Titan so they can have all the systems in place to reduce the massive cost of developing the beast from the ground up for just one product. In fact (err, speculation?), they could be testing much of the same technology right now in Project Titan. Why not develop the system in two products instead of one, thereby reducing the development costs, and along the way, create ways to get money into the system and maybe keep it there (Blizzard Balance), create good relationships with third-party internet money machines (PayPal, and incidentally, eBay), and put a stop to Chinese gold farmers? Instead of being a liability (like old school Diablo BNet and WoW AH), RMAHs will be an asset!
That just seems logical to me.