Marc Ambinder makes some good points in his post "In Defense of Double Standards, Sometimes" -- and one big mistake. I'll refer you to his site for the good points rather than attempting to paraphrase them.
Here's the big mistake: Ambinder isn't really defending "double standards."
It does not follow that similar incidents should be treated similarly, particularly if the magnitude of the differences are more significant than the similarities. Double standards are often defensible.
And here's the problem: If "the magnitude of the differences are more significant than the similarities," it isn't a situation where the phrase "double standard" is appropriate.
The phrase "double standard" means that two identical (OK, nearly identical) situations are being treated differently.
It doesn't make any sense to apply the phrase "double standard" to situations that have greater differences than similarities. It's like saying we have a double standard in the way we punish murderers and jay-walkers. Well, no: We have different standards for murderers and for jay-walkers, because they have done vastly different things.
Now, I'm sure some of you are thinking "OK, but isn't this just semantic nit-picking?"
No. When we use the phrase "double standard" in discussing disparate reactions to dissimilar events, we suggest that the events are not dissimilar. We blur the differences -- and, in doing so, we advantage the perpetrator of the greater misdeed.
Take a look at the reason this discussion has come up: Republicans (and many media figures) are saying or suggesting there is a double-standard in the way Democrats and Republicans are treated when they make racially-charged comments. The basic argument is that Republican Sen. Trent Lott lost his job as Majority Leader when he made such a comment, while Democrat Harry Reid has not lost his job. (A variant of the argument: Democrats -- and the media -- were more critical of Lott than Reid, so they have a double-standard for Democrats and Republicans.)
Now, let's review the two situations: Trent Lott suggested America would be a better place had we elected a white segregationist presidential candidate. Harry Reid used archaic language in talking about the black man whose presidential candidacy he supported.
Those are not the same things. They aren't even close to the same things. One is pretty clearly much, much worse than the other. And so the phrase "double standard" does not apply. In using it, rather than describing exactly what each man said, the media blurs the difference between their comments, suggesting they are the same (or, at least, equally bad.) They confuse, rather than clarify.
Like I said: Ambinder makes some good points. But he isn't really defending double-standards. He's defending treating different situations differently. One of the lessons journalists should take from his post is that the danger in habitually describing things as "double-standards" just because one side in a given dispute wants them to.