They've never explicitly said "Yeah, we built to game to try to juice as much out of you through the RMAH" if that's what you're asking. It's pretty clear if you actually played the game, though. The game was tuned, both in terms of difficulty and drops, with the expectation that the player would be using the AH[1], and Blizzard built the RMAH in order to attempt to capitalize upon the sales that previously happened through D2JSP or whatnot.

It's exceptionally clear that Blizzard's idea was to control scarcity of desirable items so that people were incentivized to buy and sell these most desirable items for real money, giving Blizzard their 15% cut on every sale. We have a scenario in which the same entity that brokers sales (for a fee!) is the same entity that (arbitrarily) controls the production of items sold -- it doesn't take an MBA to connect the dots there.

No, I can't prove any of this. I would be exceptionally surprised if anyone could. But companies of Blizzard's scale don't do things like a real-money marketplace (from which they take broker's fees) just because "the community demands it" - that absolutely was the monetization strategy for the game. People don't pay a subscription fee, or purchase microtransaction items in the traditional sense. How else are you going to monetize it?

(They had a similar monetization strategy for Starcraft 2, IIRC. Something about a marketplace for community-created content that, again, they would take a cut of. Let the players do the work, take a cut of it. I don't play SC2, so I'm not sure how this shook out.)

[1] http://i48.tinypic.com/2cwsg3q.png (sorry, original thread is now gone)