That's similar to what happened to me when I got my macbook. I got a free 4 gig nano (after rebate) which I sold on ebay and made at least 150 (I don't remember exactly how much) which I used towards the purchase of my ipod touch. Which is currently listed on ebay as well after the recent purchase of my 16 gig iPhone
If I would have bought mine before the price drop, I probably would have spent it as soon as I got it.
On what, I don't know. Depends on when I would have bought the phone, I guess...if I bought it before the 2 week cutoff and only got $100, I probably would have spent it on a couple of accessories or something...but if it was the full $200 difference, I probably would have gone ahead and bought a Mac. Even though I couldn't have possibly afforded the phone before the price cut, it's nice to dream about what I woulda got.