Wouldn't most of the [cash] incentives already be factored into the purchase agreement that you theoretically signed? When I put in the factory order for my terrain, I had to sign a purchase agreement (which basically states that I intend to buy vehicle X with configuration Y at price Z). All of the incentives at the time (December $1000 holiday bonus, etc) are all factored into the price that I signed off on. Whether or not the dealership can claim those rebates when the actual sales goes through sometime in February is between them and GM. But I signed off on cash price "$X plus destination fees plus GST".
Granted, non-cash incentives might not stick. My dealership warned me of this very point when I factory ordered, mostly relating to GMAC financing rates. They specifically mentioned that they couldn't guarantee any currently advertised rate, and that GMAC is very tight on this policy. If my car comes in March, I pay the March rate, period.
GM's obviously still not setting sales records though, so more incentives will certainly come down the pipe.