I think I read somewhere that the command sequence has changed for the 3GS... I'll see if I can find that article again...
I tried the screen save on the 3Gs and it works fine, no difference.
The command sequence that is different on the 3Gs is the "force-quit an application" sequence. Before you just held down the home button for 10 seconds or so. Now I think you hold down the sleep/wake button until you get the power off slider, THEN you hold down the home button. Holding down the home button by itself is now Voice Control.
There may be other differences, but that's the only one I've run across.