If I knew it as fact that the 40GB PS3 would have BC by this fall, then I would probably go pick it up primarily as a Blue-Ray player with that $100 gift-card deal.
However, I don't think it will have that much affect overall, because a lot of people still don't realize that the 40GB is not backwards compatible. A friend of mine picked up a 40GB with GTAIV and didn't know until I told him. I'm sure it would give it a boost, but probably not that much.
I think it was more crucial not to drop it in the first place, as that just gave the PS3 bad press, and deterred some buyers.