Is my iPhone 3GS an MC model (tethered jailbreak)?
You may be wondering if your iPhone is an MC model. Redsn0w 0.9.6b4 will jailbreak all iOS 4 devices, however if you have an MC model (new bootrom) the jailbreak is tethered in its current version. This means that any reboot to a jailbroken state requires your iPhone to be connected to a computer.
All iPhone 4 hardware requires a tethered jailbreak, while all iPhone 3G models can be jailbroken untethered. iPhone 3GS comes in MB and MC models.
To check if your iPhone 3GS is an MC model (and requires a tethered jailbreak) navigate to Settings -> General -> About -> Model. If your model number begins with an MC your iPhone jailbreak must be tethered. MB models can be jailbroken untethered.