It's not cheap, but I've used Mr Simcard dot com to get a T-mobile sim with unlimited data for my iphone that can last either 15 days for $60 or 30 days for $80. The drawbacks are you don't have your own number for that time and the network is Edge only, no 3g. Also, you need your phone to be unlocked of course. It's overpaying a little, but there are no surprises at the end and that piece of mind is worth something.
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