Sources at Apple have told the Boy Genius Report that a revised iPhone 4 model is currently in the final phases of field testing before release. This model revision is known as "iPhone3,2" and may be launched ahead of Apple's typical summer iPhone refresh cycle. Speculation is rampant over exactly what revisions are coming to the iPhone 4.
The leading contender is a CDMA-compatible version of the iPhone 4, which could be offered by either Sprint or Verizon Wireless in the US. Although this would make sense given the impending end of AT&T exclusivity, BGR also reports that the new iPhone 4 retains a SIM card slot. SIM cards are used on GSM networks like AT&T and T-Mobile.