There have been many interesting arguments on both sides of the issue.  What is missing is some empirical data to back up these arguments.

Does anyone know where to find a published study on the readability of single spacing versus double spacing?  How does a typical audience respond to each style?  Do they respond differently to spacing in printed versus online text?  Has anyone performed any actual measurements of reading time, comprehension, and audience willingness to read single-spaced versus double-spaced text?

Personally, I'm a double spacer, but I'm willing to change if someone can show me some real data "proving" that one way is better than the other.