According to MacNN, an iPhone with both CDMA and GSM modes have been detected. According the original link, Techcrunch, an iPhone was detected using both AT&T and Verizon networks. However, this isn't surprising given the rumors for months that the next iPhone will be able to work on both more networks using one iPhone model - both GSM and CDMA.
Although that was expected, it seems a bit late in the cycle for just such an iPhone to appear. Considering, this should be have seen a while back.
However, such an iPhone does mean that it will most certainly not support LTE. You can't have it both way, apparently.
Source: Techcrunch via MacNN.