Now, just call me an old fuddy-duddy, but I have to confess something.
Event though I've been told that the new rule is to single-space everything you type (keyboard?), my fingers don't always remember. I was taught on a typewriter, where the rule was to double-space between sentences and after colons, because it made it easier to read. And now my manuscripts seem to be a combination of both, depending on how fast I'm typing and whether my fingers remember to single space or not.
I also thought that the reason publishers were now requesting single space was because they could save money and print books with less pages that way.
It turns out that the single-space rule isn't that firm -- there still is some discussion about it.
Check out these articles and blog posts:
Why two spaces after a period isn't wrong on a website called Heraclitean River.
The Punctuation Conundrum on Kathryn Gagalione's blog, Hers for the Reading.
and from the MLA rulemakers, themselves :