[GadgetKen]

New AT&T prepaid data rate structure as of (approximately) Feb. 3rd, 2009 for prepaid Medianet(per an AT&T CSR who gave limited details on Howard Forums)

By the kilobyte at one penny a kilobyte
1 Mb data package at $4.99 good for 30 days; up to maximum of three packages may be stacked
100 MB data package (best for qwerty keyboard AT&T dataphones like the AT&T Pantech Slate or the AT&T Tilt) at US$19.99 good for 30 days; up to maximum of three packages may be stacked. Note that AT&T now officially supports a Qwerty AT&T GoPhone by introducing the Pantech Slate on their website (currently US$119).

The 1 Mb and 100 Mb data packages can be elected by dialing 611 and saying “buy features”. The penny per kilobyte rate is the default rate and you don’t have to do anything to get that rate, but it's not practical for laptop browsing.

What you could do is buy an inexpensive AT&T gophone from a department store and activate it (nice ones for only US$20). Buy a US$100/1 year card and apply it to the phone (you will also get some bonus airtime for doing this). Then just buy a 30 day data pass for 100 Mb on the cellphone by dialing 611 when you need it, and then transfer the sim card to the phone you are tethering from. Works out to US twenty cents a megabyte. This would be appropriate for occassional light tethering, but a data card and data plan would be more appropriate if you wind up exceeding 100 Mb frequently.

A non-gsm prepaid data alternative is Slingshot Prepaid Mobile Broadband for an EVDO Sprint/Verizon USB data stick. Best if you intend heavy data useage (US$60 buys 5 gb on network or 300 Mb off network).