Just restore in iTunes to get back to factory state.

To use elsewhere just redsn0w then UltraSn0w and enjoy. Data prices on PAYG can be expensive - make sure you know what you are getting.
eg in the UK on Voda and O2 you pay for whay you use but on Orange and T-Mobile they cap it at a max of £1/day - the difference being that you have to request that feature each day from orange and you get charged £1 for that day, but with T-Mobile it is charged on usage but if you reach £1 for that day then you don't get charged any more regardless of usage, you don't need to request anything.

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He who asks a question looks foolish for 5 minutes. He who doesn't ask a question remains foolish forever.

Officially unlocked iPhone 3G FW 3.13 RedSn0wed.