i have just got an iphone 3g from Softbank in Japan.

I wanna know that if i jailbreak my iPhone 3g with firmware 2.0, is it possible to get back to unjailbreaked version? As i have heard a lot of Softbank iphone users saying that after jailbreaking, they are not getting the signals.

Is there any Softbank User here who has jailbroken iphone 3g?