1. Unplug your iPhone 3g[s] from the computer and power it down.
2. Wait 5 seconds after the phone is powered off and hold down the Home button.
3. Continue to hold the home button and plug your iPhone 3g[s] into the computer.
4. Continue to hold the home button….
5. When the Apple logo disappears and the Plug into iTunes screen appears release the home button.
You have successfully put your iPhone 3g[s] into recovery/DFU mode and your phone will now be recognized by itunes as in recovery mode.:D