Thanks to Caroline for sending me a CNN article on dude. Extract below:

An admitted dude-user during his college years, Scott Kiesling said the four-letter word has many uses: in greetings ("What’s up, dude?"); as an exclamation ("Whoa, Dude!"); commiseration ("Dude, I’m so sorry."); to one-up someone ("That’s so lame, dude."); as well as agreement, surprise and disgust ("Dude.").
Kiesling says in the fall edition of American Speech that the word derives its power from something he calls cool solidarity — an effortless kinship that’s not too intimate.

Cool solidarity is especially important to young men who are under social pressure to be close with other young men, but not enough to be suspected as gay.

Language Log also has very interesting things to say on the subject (click here, here, here and here.

I’ve come across dude when subtitling American films/TV series, and I often chose to not translate it. I just don’t think there is an appropriate equivalent in French. The best solutions I can think of, in some cases, are mon pote or mon vieux, but these two expressions just aren’t as flexible in their meaning as dude, which can be used to express a variety of emotions. For a start, you couldn’t use them on their own, they generally end a sentence : "Ça va, mon pote ?" (are you ok, dude?), "Tu veux aller boire un coup, mon vieux ?" (fancy going for a drink, dude?).

Once a proofreader inserted a mec (bloke, dude, guy) in my translation. I just don’t agree with that. I know that’s what the dictionary says, but it just doesn’t sound natural, I’ve never heard a French person use mec in the same sense as dude. As this word has only ever cropped up in my subtitling work, I have no qualms in not translating it when it is part of a sentence, as it is difficult enough to convey crucial information in the limited space allowed for subtitles. Besides, dude often indicates a certain level of familiarity between people, and this is conveyed on the screen. When it is on its own and used to express an emotion, I chose an equivalent interjection in French : for example, Ça alors ! (my goodness !) to express surprise, Tu plaisantes ? (you’re joking ?) to express incredulity, etc.

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UPDATE

Following a Language Log post on this post, I realised that when I gave equivalents of “dude”, like “ça alors !” (my goodness !) etc., I concentrated solely on translating its meaning and not its exact “dudish” level of language. Not a very good thing to do for a translator, but finding ways of conveying cool solidarity in French would have required a lot of research (I am 31 and these things no longer come naturally to me – wait – they never did!!) and I just didn’t take the time to do it. I’d be really interested to hear what English and French interjections people would use to express surprise or incredulity using “cool talk”…