Rob, I thought about the same thing when I recently bought an '07 Explorer. I'm sure some folks with more financing knowledge will be more helpful than I in detailing the finer points, but here's how I understand it. Ford's own financing company offers one or the other. The people with whom you negotiate the selling price of a car don't represent that finance company. Think of Ford's finance company as a "preferred lender", nothing more. What seemed to become clear to me was that the financing incentives were not negotiable. If you take one incentive, you don't get the other.
BUT...you get to apply whichever one of those incentives you choose to the best deal you can make. In other words, if you are fortunate/effective enough to negotiate a deal for $5k below invoice (I know that's absurd, but it helps make the point), then you still could choose to knock off yet another $1,500 by taking the cash back option.
If you're convinced that the 4.9% financing option is the one you want, then you just have to make your best deal possible and then apply the special interest rate financing to that deal.
I guess my whole point is, the dealer doesn't have anything to do with the incentives offered by Ford's finance company, which basically means the only back-door way to accomplish what you're thinking about is to wait until you've gotten to where you like the deal, and then try to negotiate the price another $1,500 downward before signing on the ol' dotted line.
-JD