Two days ago we reported to you how to jailbreak your iPhone 3G/3GS, but a very serious side effect has occurred - a lot of people who did the jailbreaking exercise say that not only their devices have poor battery performance, but also the fact that their iPhones become extremely hot when in use.
A permanent cure for these most unwanted side-effects is being developed at the moment and when ready, it will be officially released on Cydia. Until then, a temporary fix is available - if you have an official iPhone SIM, all you have to do is to activate it, take it out and go back to the version you want. For more information, check the link below to find a temporary way to limit the damage.
source: Redmond Pie