It is crucial that the 3G antenna bracket is mounted back in the right order.
I am talking about this part (the image attached).
[http://s11.postimage.org/531tgtov7/bracket.jpg]
This is the order it should be reassembled:
1. Iphone motherboard
2. Speaker housing with 3G antenna (which _alot_ of people think is the Wifi-antenna) (bottom of the Iphone 4 anyway...)
3. The 3G antenna connected to the motherboard (the small round cable)
4. The 3G bracket (the part in the image attached)
5. The battery contact cable
6. The correct screw that keeps things in place.
Enjoy!
Also, clean metal parts with something like Windex.