One thing I know about Apple, assuming that Apple US is the same as Apple Aust., is that they HATE hurting their retailers. How would they do this; creating too much inital demand for the new iMac will surely have FAR too much impact on the market. As it is, sales for older iMacs and current G4s will be plumeting but resellers and distributors still have to move their stock.
Also, we all know how much Apple holds their closest secrets in a tight grip covered by so much red tape you would need a chain saw to get through so there is no chance of seeing it. I would gather that they would not get ads made up of things as important to the company as this new iMac until it is released to reduce the chance of leak.