I was messing around in Wikipedia which re-inflamed my interest on this subject.
Before I add my own two cents, I'll link two pieces, one arguing each side (I think it may have been arranged as a pseudo-debate but anyway): Single, Double.
If you're like myself, then I'd recommend reading all three of the linked pieces, but I'll try to run down some basics:
Typographers allegedly universally insist on single spacing after sentences. Often, a large justification for this is that double spacing was merely an ugly necessity of the typewriter era, and modern typefacing, with letters that fit together nicely, don't need that extra space to make the ends of sentences obvious.
This claim -- that double spacing was essentially invented only when typewriters were, and should die with them -- is perhaps not exactly a lie, but is certainly a gross misrepresentation of the facts. Wikipedia's historical information indicates that in the pre-typewriter era, sentences (in most styles) had a single "em-space" after them: a space about the size of that typeface's "m", like the "em-dash" that more or less survives today (though many people often "double-hyphen" instead, either out of laziness, as I did above, or ignorance). But I digress: sentences got an em-space, while words were separated by a mere 1/3 or 1/2 of an em-space. Therefore, the difference in practice between the pre-typewriter em-space and the modern double-space is minimal at best. This false accusation is not pandemic among single space advocates, but I've seen it enough to include its debunking.
I've heard other justifications for the single space rule, from "they’ll throw-off spacing in professional page layout programs" (IMO this indicates either a crazy claimant or a crazy program) to "a page of text with two spaces between every sentence looks riddled with holes" (blatant personal opinion).
For the other side, I'd really suggest the above link, but if you're detemined to read only my own words, it boils down to the fact that single spaces don't sufficiently indicate the end of a sentence -- periods aren't a reliable indicator, and I'd like to see you disagree! Unless you put double spaces after colons, their exclusive function is to serve as such an indicator (and you can [almost] only put a colon where a sentence could end anyway). I've even seen an argument that since periods have a sort of virtual space above them, that's almost as good as double spacing. I have to call bullshit on that, plain and simple: commas occupy the same space, while exclamation points and question marks don't.
Another argument can be made in favor of double spacing on the theory of pauses. If you imagine someone speaking the sentence, generally you imagine a certain pause between words unlessyouimaginethemtalkingreallyfast, and a certain pause after a comma, or a semicolon; a period gets a longer pause than a comma: but colons and semicolons get the same pause as a period, which undermines my point a little. Maybe I should start double spacing semicolons (I doubt I will).
tl;dr: I double spaced every sentence, and colon, in this post -- not that you'd ever know, thanks to those damn browsers.
[edit: Oh shit! I just checked and I see double spaces! Awesome! I take it all back, Firefox, I love you. Also, everybody who read the whole OP ought to read Eats, Shoots & Leaves which is a beyond-awesome book about punctuation. Sounds like a joke, I know, but it really is.]
Tag (courtesy of fkusumot): "Please feel free -- nay, I encourage you -- to offer rebuttal."
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My advice to fanboys: Brag about stuff that's true, not about stuff that's false. Predict stuff that's likely, not stuff that's unlikely. You will be happier, and we will be happier.
"Everyone is entitled to his own opinion, but not his own facts." - Sen. Pat Moynihan
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The old smileys: 
; - ) : - ) : - ( : - P : - D : - # ( c ) ( k ) ( y ) If anyone knows the shortcut for 
, let me know!
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I have the most epic death scene ever in VGChartz Mafia. Thanks WordsofWisdom!
