1. Cost of goods sold reduction. You have to remember this is a public company and profitability is the number one concern for the "owners." Thinner computers use fewer raw materials to produce computers with. This allows apple the advantage of maintaining price points, retaining product line margin. In some cases apple might be able to reduce selling prices on products and not erode too much off their margin too.
In the UK, the company which popularized the home PC was not IBM, but Amstrad. The boss there, Alan Sugar (yes, him off the Apprentice) looked at the way the hard disk was fitted into the PC (four screws - one at the front, one at the back on either side) and had them remove two screw so one side had 1 screw at the front, and the other side had one screw at the back. This didn't cause any stability problems, but saved 2p per machine. One million machines sold later and you've increased your profit by 10 grand.
So in principle I agree with your hypothesis, but when it comes to case manufacture, you're talking about retooling the production line... to do this merely to have a slightly thinner iMac (which would not be a reason for 50% of the current iMac owners to upgrade, really) seems a little over the top, maybe? I'd expect some sort of killer new function to be the selling point, and the thinner enclosure to have been put in as added value.