Via this week's Blawg Review, I learned of AdamsDrafting, which has this post siding with the one space camp, of which I am a member.
It says in part:
"The Chicago Manual of Style 2.12 (15th ed. 2003) says 'A single character space, not two spaces, should be left after periods at the ends of sentences (both in manuscript and in final, published form) and after colons.' To my mind, that settles it, but I’ll note that The Associated Press Stylebook (2004) also calls for one space. So does Bill Walsh’s Lapsing into a Comma (2000). . . .
Of course, law firms and most lawyers are wedded to two spaces. It would be a mistake to assume that this is the result of a reasoned decision. Instead, you can attribute it to the same oblivious conservatism that has caused them to perpetuate any number of other deficient usages.
As the online Chicago Style Q&A states, there's no evidence that using two spaces makes text easier to read. Consequently, the only conceivable defense of the practice is that it's harmless. But as also noted in the Chicago Style Q&A, using two spaces is inefficient, requiring an extra keystroke for every sentence, and is harder to control, in that any document created using the two-space rule is likely to contain a 'a smattering of instances of both three spaces and one space after a period, and two spaces in the middle of sentences.'
So if you're still using two spaces, stop it - your credibility is at stake!"