National Geographic partnered with Cellular Abroad in order to offer the handsome Duet D888 dual SIM phone. Made by Beyond E-Tech, this unlocked handset can host a GSM SIM and another card of your choice.
Duet is a silver candybar phone with mediocre features, like a 2 megapixel camera, an MP3 player, FM radio, Bluetooth, USB, messaging, a speakerphone and a personal organizer. National Geographic content is also on board (wallpapers, ringtones) and you should know that the phone is not flawless, since it lacks support for the GSM 850 band, used in North America.
This may be a triband phone (GSM 900/1800/1900), but getting it to work on the 1900 band in the US can be tricky.