No Sim Card Installed - iPhone 3GS (iOS 5.0.1)I will list out the horror story later. Basically if you have a "No Sim Card Installed" or "No Service" message. Do the following exactly. Do not read into the instructions - just perform the steps.
1. connect iPhone to Mac with iTunes.
2. remove the SIM CARD (take a paper-clip and push it in the top pinhole to eject the card).
3. put iPhone in DFU mode (that is holding down the POWER BUTTON & HOME BUTTON both at the same time for exactly 10 seconds. Release the POWER BUTTON only).
[a message on iTunes shows that an iPhone is connected in recover mode.]
4.click RESTORE in iTunes (this will restore the iPhone to the stock/normal firmware.)
5. wait til it is finished.
6. when the phone boots, it will show "No Sim card." Don't do anything yet.
7. simply put the simcard in the top SIM CARD slot.
6. the phone will activate the phone automatically.
Update: 11/03/12: This is one of the most popular articles on this web site. Please note that this info is provided as in my own experience and it isn't meant to fix your particular issue. Please make sure you've backed up everything. Please make sure everything is synced. Please refrain from emailing me, asking me for support. If you want my advice, upgrade your phone to the latest version available (iPhone v5) and have the store/service transfer everything.