This is a great topic, related in many ways to so many other format decisions, such as what type of font to use, whether to underline or use italic, etc. I came across this "Guidelines for Briefs and Other Papers" from the homepage of the Seventh Circuit a few months ago and used their suggestions for our moot court brief. http://www.ca7.uscourts.gov/Rules/type.pdf
The suggestion (not requirement) to use 1 space is located toward the bottom of page 5 of the document.
Overall, I find it geekily interesting. They mention that the judges read about 1000 pages of text for each argument session, so it is to one's best advantage to make the document as readable as possible. Everything from using a book-like font instead of newsprint (times new roman, e.g.), because the latter is designed to read quickly, which hinders comprehension, to not using italics instead of underlining for case names, etc (noting that court reporters use italics and underlining is for typewriters...since computers can italicize, why not take advantage of it).
Anyway, I like most of the suggestions. I like the 1 space suggestion, too, but after years of typing with 2 spaces, I just can't do it no matter how hard I try.