Yes, the iPhone 4 can be purchased without a contract. Unsubsidized prices are at a $400 premium without an AT&T Wireless contract.
This puts pricing at $599 for the 16 GB iPhone 4 and $699 for a 32 GB iPhone 4.
Users who aren't eligible for an upgrade may want to purchase the iPhone 4 at full price. Others may prefer the flexibility of no-commitment wireless service that allows for cancellation at any time without the $325 early termination fee.