A friend of mine posted a link to this Slate article about why you should never use two spaces after a period. I thought it was great! Finally some anger on the side of the (obviously correct) type designers and typesetters!
My theory is that there's a sizable age gap in this debate. I think people have a very hard time changing their habits and anyone who learned to type on a typewriter probably still considers it correct to use two spaces. My guess is that most people under 40 (and even more people under 30) just use one space. I would love to see statistics on this but I don't think it's too high on anyone's priority list.
Anyway, since we all use computers and not typewriters let's stick to one space. Please!