AT&T (formerly Cingular) has a prepaid SIM to which mobile web browsing options can be added. Currently there are the following options:
1MB = $4.99 / month
5MB = $9.99 / month
100MB = $19.99 / month. For details see here. AT&T stopped the unlimited plan in Nov 08 and only offers a pre-paid 100MB option for $19.99.You activate this multiple times a month.Tested and works with unlocked iPhone in June '09.
Misc information:
The options are activated by calling 611 (a voice recognition system). Select "Feature Packages" and then "MediaNet".
According to this link, the APN to use for the offer is wap.cingular
The Pay-As-You Go offer is described here, but information on the MediaNet options is scarce.
All add-ons seem to be for mobile browsing only, but the phone can be used as a modem for a notebook (last checked June 2008)2008). iPhone tethered to laptop with the 100MB plan and APN listed above with no issues. Only http and https were accessed though, so other protocols are unknown. (June09)
The ATT homepage does not mention if other services suchs as POP/SMTP, etc. are available. In practice, POP/SMTP and also IPSec for VPNs is possible (last checked June 2008).
Network: GSM/EDGE in 1900 and 850 MHz, 3G in same bands but not sure if Prepaid SIMs are allowed there.