United States

Default data rate
$10.24 / mb. See link for more information.

Data feature packs
$4.99 / 1 MB.

$19.99 / 100 MB See link for more information.

Availability
AT&T SIMs can be purchased in their own shops

SIM sizes
Mini SIM's exist. Micro SIM availibility unknown.

iPhone specific information
The AT&T sales reps can be reluctant to sell you a pay as you go plan if you have an iPhone.

Once you have the SIM you need to set up your iPhone properly, described here.