ok nice got it, so here's what I did:
1. Turn off your existing phone completely, in my case it was an original iPhone.
2. Then make sure your iphone 3gs is disconnected from the computer.
3. Turn off your 3gs
Both phones should be off at this point.
4.Turn back on your 3GS and it should be activated
Hope this helps.
worked for me thanks for the fix!!