According to iDealsChina the iPhone 5 will not only hold one, but two SIM cards to support dual phone lines. This sounds very odd and I think iDealsChina could have misinterpreted the information from their source.
The iPhone 5 is rumored to include a Qualcomm just like the Verizon iPhone that allows for both GSM and CDMA connections in the same phone. Perhaps this is what they mean by dual SIM-cards?
iDealsChina, a Chinese parts reseller, is the site that brought us iPhone 5 design images. The site has a mixed track record of correct & incorrect rumors. They incorrectly reported that an iPhone nano would launch in 2008.