How do I restore a jailbroken iPhone 3G?
All you need to do is plug your iPhone into iTunes, click on it under Devices:
And click "Restore":
iTunes will make sure that you do actually want to restore:
Jailbreaking does not make any permanent changes; a restore will put your iPhone back exactly the way it was before you jailbroke it.
There aren't any permanent changes made to your iPhone when JailBreaking. All you need to do is plug it into your computer, open up iTunes, and update/restore iOS normally. This will return it to a fully working, non-jailbroken state.