How many spaces after a period at the end of a sentence?
One or two spaces between sentences?
Answer
The Publication Manual of the American Psychological Association, 6th edition, does not offer us a "smoking gun" on this question. However, I think we may be able to infer the answer.
On p. 88 the manual states ""spacing twice after punctuation marks at the end of the a sentence aids readers of the draft."
I would read this as having permission to "sin" with two spaces in the rough draft.
Ergo: Only one space after a sentence ending period in the finished work.
See also: http://css.libanswers.com/a.php?qid=264348