Please note that this story is based upon rumor and/or speculation.
Apple is planning to sell unlocked iPhones online and in Apple stores, according to 9to5mac.com. The 8GB iPhone 3G is rumored to run for $499 and the iPhone 3GS will set you back $599 and $699 for the 16GB and 32GB models, respectively.
Despite the rumor that this could be imminent, and a confirmation from Engadget, Apple's online store still has this listed in its FAQ:
"Q: Can I buy an iPhone without an AT&T contract?
A: No. iPhone requires a two-year AT&T wireless service contract."