I think that the "production delays" are part of the marketing strategy.
By creating a perception of scarcity they feed into irrational impulse driven demand.
Haven't you noticed how apple devices always "sell out"? They are produced in quantities designed to do so.
The iMac is no different. Hype drives sales. Perceived scarcity builds hype. Very simple.
It makes the people that rush out and blow 3 grand on a desktop computer feel superior for a few days/weeks/months.
I'll tell you when I ordered my rMBP the experience was all the more exciting because I had to wait to have it delivered, and was the only person I knew that had one.
Now you can walk into best-buy and buy one.... it's lost it's exclusivity