well, first i called at&t with questions about what i was doing. i called once, got some answers then got disconnected... called back, got a different set of answers... oh well.
but... they told me that i
had to activate my iphone through the itunes store. it was super simple and i was up and running in a few mins. i think on the first screen, i just selected "activate new service" or something. i'd guess i'll pay like $69 or something + tax for the month if i decide to cancel before 30 days.
i'm guessing that they wouldn't let you get service, then cancel, then get service again right away. one CSR thought that if i canceled and got service right away, i might incur some deposit. the other one didn't think that would be the case, however.