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The 2G's screen is narrower than the 3G. So they will not cover the entire 3G screen. Not sure if the holes line up either.
 
I just got my 3GS last night to replace my 2G and I put on one of my extra 2G screen protectors on the 3GS. The protector does not cover all of the screen, but the holes do line up... at least the one I have does.

I will be getting a SP for the 3GS after work tonight and probably a cheap case for it until I order a good one.
 
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