Generally 1 day after you buy something.
I think the 6600 will drop first as that is the older card. I think these are scheduled to happen quite soon. The 6800 may drop a little as it has been superseeded but probably no big drop for a while yet.
The AMD 3000 range is now the San Diego core so in effect these are "new" chips. Once again though these will drop then something newer, better and most likely quicker comes along. I doubt the release of the FX-57 chip will effect this range though. Might be able to pick up a cheaper FX-53 or 55 though.
This is of coruse all spectulation and we must remember that different manufactures will do different things at different times. nVidia has no say over the manufactors, suppliers, dealers and so on all who are in the chain. AMD on the other hand will decide when they lower there prices. Just prey the rest of the chain passes it on.