United States

Default data rate

 * $0.01 per 5 KB

More information

Data feature packs
You can start by purchasing the 100mb package, and if you renew with the 1mb package within 30 days you can keep your leftover bandwidth.You cannot renew using this method more than three times. After the third time, you must order the 100 MB package to retain your leftover bandwidth. AT&T PayAsYouGO Dataplan does not work for iphone* and Blackberry in the US. More information [note: I had GoPhone plan with the 15 usd/100mb option. Maybe that link is outdated]
 * $5.00 / 10 MB
 * $15.00/ 100 MB
 * $25.00/ 500 MB

Availability

 * AT&T SIMs can be purchased in their own shops. If you go to a mall kiosk, they will attempt to charge you for the SIM. Instead, go to a brick and morter AT&T store (Store locator) where they will give you the SIM for free when you put $25 on the account. Do not tell them that you have an iPhone.
 * An alternative are sites ike ebay where you can buy the SIM cards even abroad. Topping up can be done on sites like callingmart.com
 * To keep the SIM active for 12 months, you are only required to top up with a single $100 topup card (you cannot from what's online, top up with multiples of anything less than $100.

SIM sizes

 * Mini SIMs exist. Micro SIM availibility unknown.

Tethering
Will work on a GoPhone SIM provided you have purchased the data plan.

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. Say that you come from another country (like Italy, Belgium, Canada) where iPhones are sold factory unlocked. That argument usually works.
 * Once you have the SIM you need to set up your iPhone properly:
 * Create a custom APN on a Mac/PC computer using Apple's (free) iPhone Configuration Utility, as described here.
 * Or, if you have wifi access you can download a pre-prepared APN directly to your iPhone by visiting http://www.unlockit.co.nz in the phone's web browser. Choose: Custom APN > Carrier = US - AT & T > Create Profile, then install it.
 * More instructions for the AT&T GoPhone: http://theappleblog.com/2010/07/26/how-to-give-your-old-iphone-new-life-with-prepaid-data-and-minutes/

Pay Per Day
A pre-pay SIM, available in stores. Only pay for the days you use.

Data rate

 * $2 / 24 hours (2G Speeds)
 * $3 / 24 hours (4G Speeds)
 * Unlimited data
 * Unlimited talk
 * Unlimited text

T-Mobile Pay By The Day plans

Availability
Can be purchased in store, no ID needed. SIM costs $10 (comes with ~$3 credit).

Speed
4G & 3G speeds require your phone to work on T-Mobile USA's frequency. If you have a European phone, you can probably only get 2G.

4G speeds are for the first 200MB per day.

Web DayPass (works with iPhone)
For t-Mobile TO GO prepaid clients

Data rate
T-Mobile Prepaid plans
 * $1.49 / 24 hours
 * Unlimited data

Data rate

 * $70.00 / month
 * Unlimited data, voice, and text

More information

Unlimited data, voice, and text. Accounts can be terminated at any time at a T-Mobile store, by phone via My T-Mobile or online chat support with a T-Mobile consultant.

You should sign up for your My T-Mobile account whilst in the US because the password is sent via SMS which is not accessible when roaming outside of T-Mobile coverage.

Availability
Store locator
 * T-Mobile stores.

SIM sizes

 * Mini SIMs exist. Micro SIM are not officially supported by T-Mobile US - SIM Cards need to cut a special Cutter or by hand if you feel comfortable to do so.

Tethering
iPhone
 * Yes.
 * T-Mobile does not sell and does not support iPhone (at least not in April 2011)

Virgin Mobile
Broadband2Go (No Annual Contract)

Data rate
You must buy one of the devices (MiFi or USB Stick). The MiFi costs $149.99, the USB stick is at $79.99
 * 100 MB - $10/month
 * Unlimited data - $50/month

NOTE: Virgin Mobile reserves the right, without notice, to temporarily limit throughput speeds when monthly data usage on the $50 Broadband2Go Plan exceeds 2.5GB.

http://www.virginmobileusa.com/mobile-broadband/broadband2go.html

Availability

 * Store Locator

SIM sizes

 * unknown

Tethering

 * unknown.

SouthernLINC Wireless
Monthly Unlimited plan:

Data Rate:
 * Unlimited data - $50/month.

Avaliable on: http://www.southernlinc.com/prepaid

SIM Sizes
 * Unknown

Tethering
 * Unknown

Plans
More information
 * $40: Unlimited nationwide talk & text
 * $50: Unlimited nationwide talk & text + 100 MB
 * $60: Unlimited nationwide talk, text & web

Availability
Store locator
 * SIMPLE Mobile SIMs can be purchased in their own shops or online.

SIM sizes

 * Mini SIMs exist. Micro SIM don't exist, but local dealers often have a micro SIM cutter.

Tethering
Unknown

iPhone specific information
Warning : Iphones (v4 included) will only operate at 2G but not 3G as they does not support the 1700Mhz simple mobile network, more information

Cellular Data MMS#SAPN: simple
 * APN: simple
 * 1) Username: blank
 * 2) Password: blank
 * 1) Username: blank
 * 2) Password: blank
 * 3) MMSC: http://smpl.mms.msg.eng.t-mobile.com/mms/wapenc
 * 4) MMS Proxy: 216.155.165.50:8080
 * 5) MMS Message Size: 1048576
 * 6) MMS UA Prof URL:http://www.apple.com/mms/uaprof.rdf