How scratch resistant is the screen of the Samsung Galaxy S 2? I wonder if over time the screen of Samsung Galaxy S2 may get scratched? Is it necessary to use a screen protector?
The S2 has Samsung's toughened Gorilla Glass screen, see the videos in this answer to a previous question for attempts to scratch the Gorilla Glass on an original Galaxy S: https://android.stackexchange.com/questions/2756/looking-for-a-robust-android-smartphone/2777#2777

I can say that I've had an original Galaxy S for over a year now, and regularly keep it in a trouser pocket with my house keys, have dropped it a few times, and other slight accidents, and while the back and sides of the cover have scratches and chips on them the screen is still absolutely unmarked.

However what you might want to consider is the fact that the camera lens on the back apparently isn't gorilla glass, see this question Is Samsung Galaxy S II rear camera Gorilla Glass too?