Have you tried holding the home and power button to reset or will it not let you? I had an issue similar to this the other day where I pulled my iPhone out and it was on, but I couldn't get the screen to turn on. A quick reset for me did the trick...
However, OP did state that the phone is working, such that it can connect to itunes, he can hear the vibration.
The symptom of what you speak of is only when the phone won't turn on at all, no vibration, can't connect to itunes, his case is different, his phone works, but screen doesn't work. He can reset or restore his device and yet still get a black screen.
My sister had this issue too when she dropped her 3G, the screen turned white, it can turn off, it can connect, but the screen remained white no matter what she does to it.
OP: It's likely that the impact may have caused some damage internally such that maybe the cable ribbon got disconnected, thus your screen not working. If you are out of warranty, and you have a iPhone 4 it only costs $149.00 to replace (depending on which country you are in). If not you can open your device and see what is wrong with it. You may have to end up ordering screen replacement along with digitizer.