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Look at the "Credits" section here...there are more data providers that you can cite.
Thanks for a tip!
So, the actual reference will look like this (?):
National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, iPC, (2011). National Geographic World Map, digital topographic basemap of the world. Accessed on-line http://www.arcgis.com/home/item.html?id=b9b1b422198944fbbd5250b3241691b6 on 27.03.2012.
and the reference in a text shall be (National Geographic et al., 2011) (?).
In fact it's a good idea to provide the "How to cite" section to the basemap descriptions so people can know for sure how to reference these maps in their work; ArcGIS on-line services are becoming more and more popular each year.