Very nice utility written in VB that will take any normal bitmap, perform gnomonic projection on it and save as 6 separate bitmaps.
These 6 bitmaps then can be used as cube maps for skyboxes, environment mapping, etc - with no seams if you do it correctly.
Your original bitmap should have a 2:1 ratio.
http://www.unrealroc.com/CubeTheSphere.html