iPhone without a contract

[Krolls]

I would like to purchase an iPhone and use it with the pay-as-you go plan as needed, however there are times when I am off at school and would not need to pay for the plan. How could I "hack" the iPhone so that when I dont want to pay for service I can basically use it as an iPod touch but when I am home during the summers I can use the pay-as-you-go?

Thanks,
M