United States

Boost Mobile
Note: Check coverage map for your location(s) because Boost coverage is not as great as the larger networks.

Also, no 3G.

Default data rate
35c / day unlimited usage.

Data feature packs
Also included as part of their $50/month unlimited everything plan.

Availability
Can be purchased at most electronics and grocery chains including Walmart, Best Buy, Radioshack and even Publix and Kroger

SIM sizes
Mini and micro SIM availability unknown

Tethering
Unknown.

iPhone specific information
Requires an unlocked iPhone in the U.S.

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 SIMs exist. Micro SIM availibility unknown.

Tethering
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.