AT&T is the official 3G carrier for the original iPad in the US. Apple has struck what they consider a landmark deal with AT&T to provide service for the iPad.
AT&T offers two 3G service plans for the iPad. Both plans are no-commitment, cancel anytime agreements.
250MB of usage costs $14.99 a month.
2GB of usage costs $25 a month.
Unlimited data usage costs $29.99 a month (these plans have been discontinued, but existing unlimited subscribers can continue to renew their plan unchanged).
UPDATE: The iPad 2 with 3G is available on AT&T Mobility and Verizon Wireless. A variety of data plans are available and detailed here: