Apple may have already booked the Moscone Center for their annual Worldwide Developer's Conference. In previous years, the conference has been used to introduce new iPhone models.
AppleInsider has noticed that The Moscone Center Calendar lists a "Corporate Event" from June 28, 2010 through July 2, 2010 in the Moscone West hall. Previous Apple events have been reserved with the same title.
The original iPhone was released on June 29, 2007 and required an exclusive contract with carrier AT&T. It has since been rumored that this contract with AT&T could expire in 2010. If WWDC 2010 does take place on June 28, 2010, Apple might use the event to announce an additional carrier for the US.