I don't know if this has been posted yet, so I'm sorry if I'm repeating:
This is what the MLA guidlines have to say about it:
"Publications in the United States today usually have the same spacing after a punctuation mark as between words on the same line. Since word processors make available the same fonts used by typesetters for printed works, many writers, influenced by the look of typeset publications, now leave only one space after a concluding punctuation mark. In addition, most publishers' guidelines for preparing a manuscript on disk ask authors to type only the spaces that are to appear in print.
Because it is increasingly common for papers and manuscripts to be prepared with a single space after all punctuation marks, this spacing is shown in the examples in the MLA Handbook and the MLA Style Manual. As a practical matter, however, there is nothing wrong with using two spaces after concluding punctuation marks unless an instructor or editor requests that you do otherwise."
And this is what AP has to say about it:
"SPACING: Use a single space after a period at the end of a sentence."
I was taught MLA style in highschool English and so I used two spaces after each sentence. When I majored in Mass Comm in college my professors taught us to use only ONE space, since we follow AP guides. That is what I use now, esp because this is what my job calls for. Now when people forward me documents with the double space, it sort of annoys me because I have to go through and take them out.
ETA: It also peeves me when people list stuff in a sentence and put a comma before the word "and." Example: I want breakfast, lunch, and dinner. <-- According to AP style there should be no comma after the word "lunch."
Not sure if MLA calls for this, I couldn't find it online.
