If I was an agency compensated under what I understand to be the normal model (15% of ad spend), then I'd probably 1) accept that I'm not going to do a better job than Apple and am therefore going to lose the contract eventually and so 2) front-load the ad spend as much as possible to maximize my compensation before I lose the contract.

Maybe for the first ads, I'd try to get a really big celebrity 'name' and run double-length commercials. Try to burn through half the budget straight off the bat. If I still had the contract after that, I'd run ads where I squeezed as many clips of people into 30 seconds as I possibly could - think of the production costs. If it was coming in too cheap, maybe throw in some B-list celebrities if I could get away with it. Finally, if the client still hadn't wised up, I'd just spend whatever was left of the budget, though at that stage I'd probably only be able to afford ads where I just pull someone off the street and get them to buy a computer.

Ah - it all makes sense now. Smart guys, ad agencies :-)