I have had my iPhone 3G[s] sense it was release, that's 2 1/2 years now, My wife has the same model 6 months younger. Both batteries are pretty poor now and both our back cases have a vertical crack running up from dock connection. We have both dropped our phones many many times and not on nice soft carpet but on tile, hardwood and cement floors. These 3G[s] sure can take a beating, and no cracks or scratches on the glass. Both phones are off contract and I could just have gotten a new 3G[s] on contract for $0 or even spent the money for the new 4[s]. So why do the repair on mine? First We have moved to Florida from New Jersey and I want the flexibility to change carriers if I really need to and second I just wanted to give the repair a try. Money was never the issue, I bought the replacement battery and rear case for my phone. I also bought the Pro Tech Base Toolkit mostly because it was cool, it did however come in hand.
The Prep... I have an Apple 30" Cinema Display, on the left half I had a movie going and on the right I had the ifixit guide for the rear case replacement up. I had a 6 or so small desert plate lined up behind my keyboard to hold the removed parts in stages so as to not mix them up and making the reassembly easier. I laid down a soft cloth to perform the work on and started in. These a very tiny screws so be careful to not loose them, Take it SLOW and DETAILED, the Mac zoom feature will come in handy to enlarge the photos so you can study them in better detail because you will need to do this. The ifixit guide does have all the information to get the fix done but you may want to take some photos along the way that are of better contrast so you can see exactly how things should be. I did not do this so I spent quite a bit of time studying the ifixit photos during the reassembly process. Pay very close attention to how the volume buttons, mute switch (which was a ##&&%), on/off button and GPS antenna are laid out. Bottom line... My repair took over three hours, I really took my time. The repair was very successful, and you would be very hard press to be able to tell the difference between a new 3G[s] and mine.
Take some of your own HIGH contrast photos or video so you can see exactly how things should be situated during the reassembly. The 00 screwdriver that comes with the battery replacement will do just fine, the spudger did not, I broke both tips on mine. If you don't want to spring for the tool kit like I got then these are what I recommend. You will need the suction cup to remove the glass, The Heavy-Duty Spudger and the Metal Spudger Set will make things easier to remove i.e. the GPS antenna is glued down which is not mentioned in the guide and that is where the flat metal spudger comes in handy. Again TAKE YOUR TIME! Take breaks, eat and drink to fuel up (not in your work area) and keep everyone away, you are in for the long hall but you will be successful!