IPhone – Avoid data plan being added onto AT&T iPhone 3GS

You are probably in pretty bad shape here, the system keychains are generally created as part of the iPhone's activation process (they include things like the APNS certificate, which is negotiated with Apple during activation). Even if you do manage to create a new keychain, all the certs on it will be different, which means that anything that has exchanged keys with your iPhone (like push notifications, encrypted backups, etc) won't work.

You might be able to create an empty keychain by using the security commandline tool. I am not sure if that is actually installed on the iPhone by default. You might also be able to create an empty toolchain on a Mac and copy it over, but I am not sure if the iPhone keychain is format compatible with the Mac OS X one.

I think it is important to remind people, if you don't know exactly what you are doing then mucking with system files can be very dangerous, and you should always make sure you copy the original files so you can revert it.

You're going to have to talk to AT&T and have them remove the data plan, unless you're under contract for a data plan (as all new iPhones are) they should be able to take it off your next bill. Jailbreaking really doesn't have anything to do with this.