There’s a great thread on Media Bistro from earlier this year about breaking into NGT. Someone who had an intern there gives the inside scoop.
Media Bistro Bulletin Board: Topic: National Geographic
“I interned briefly for National Geographic Traveler magazine. They do accept queries, but not usually for large features. They have a base of well-established writers who they have used for a long time for the main editorial well. However, they do like to try out new writers in the Trips section, and I know the editor who is responsible for that section likes to try new writers when she can. That is probably your best option to break into writing for NGT. Hope this helps!” —travelgrl02