I recently saw a pic in a magazine that had 2,5 Gigapixels... 2,5 billion, just imagine... They had a different approach, a couple of cams and a comp set it together...
http://www.tawbaware.com/maxlyons/gigapixel.htm
Here guy explaines how he made his 1.09 gigapixel image, how he photographed it it, how he combined all the little images and stuff. Pretty much like making panoramas.