How do you completely erase iPhone 2G

Is there a truly secure way to completely erase my iPhone 2G of ALL of my personal data so it will not be able to be retrieved by hackers? I am upgrading to the 3G S on Friday and now this device will be unused. I just want to clear it without taking a sledgehammer to it. Just like an old computer, I don't want to get rid of it and risk identity theft.