Samsung has just announced the D880 Duos, a cellphone with two available SIM card slots. The Duos currently will not be headed to the US, but instead Europe. The D880 is a slider style phone with a 2.3-inch QVGA screen, a 3.2 megapixel camera, an FM radio, Bluetooth with A2DP support and tri-band GSM and EDGE connectivity.
Both SIM cards will be able to work at the same time giving you the option to choose which card you want to use at that time. For both incoming and missed calls you will be shown which card the call came in on and you can also set a different ringtone for each.
I am not sure how much of a demand there would be for such a phone here in the US, except maybe for a smaller group of frequent overseas travelers or possibly business users who want to use just one phone for both business and personal reasons.
The Samsung D880 Duos is scheduled to be available in Europe this November and will retail for around $700.