Double-space or single-space?

I read an article the other day in Slate that argued against the use of double-spacing between sentences. The article begins,

Can I let you in on a secret? Typing two spaces after a period is totally, completely, utterly, and inarguably wrong.

Well, they didn't exactly leave any doubt as to where they stand.

I realize this is a pretty trivial subject, but it's something that anyone who writes has to wrestle with.

The old school way of doing things is to use double-space. That's how I was taught to type and is how I did things until sometime in the last year or two when I was called out on it by Bill. I realized pretty quickly that there really was no reason to put two spaces between sentences and it wasted a lot of time anyway. Of course it didn't waste a lot of time at once, but over time, all of those extra clicks add up.

But I'm curious, what do you do and why? I'm guessing most of us use one way or the other based on what we were taught, but I wonder if there are any other reasons or if it's something you've never even considered?