From Sheldon Brown:

"Spoke length is measured from the inside of the elbow to the very end of the threads, most usually in millimeters."

If you go to this page:

http://www.sheldonbrown.com/rinard/spocalc.htm

and download Spocalc, it contains just about every rim and hub in it. You can then punch in the numbers and it will tell you what length spokes to get. It's a very handy program!