Recently, I've been reading about page layout and readability. Many web twits hold forth about various rules of thumb: alphabet and a half = 39 characters, newspaper article width = 50-60 characters, 20 times font height, I typeset my website by hand and only have enough type for 44 characters, etc.
Reading speed serves as a good proxy for readability (the more readable something is, the faster you can read it). Wichita State University ran a study on reading speed of various width lines (35, 55, 75, 95 characters per line). Guess which won?
The 95. That tells me that the biggest hindrance to reading is the eyeball carriage return. Less carriage returns means fewer number of eyeball bounces back to the beginning of the line.