Boy Genius Report is claiming that starting March 26 AT&T will being offering iPhone 3Gs contract-free to existing customers. The information which appears to have been sourced from internal AT&T training material which states that the 8GB iPhone 3G will sell for $599 and the 16GB for $699.
AT&T iPhone
The offering is only available for existing AT&T customers and does not require a service agreement or device activation.

The training materials go on to explain that customers purchasing no-commit iPhones will be limited to one purchase.