I doubt that anyone would be more amused at Cobain making the top 20 than Cobain himself, were he alive. I remember that shortly after Nevermind broke, Guitar Magazine interviewed him: his answer to pretty much every tecnical question was "I dunno" or "wow, that's all guitar geek stuff." He seeemed pretty clear about where he was playing-talent-wise, and mostly bemused by all of the attention.

While it's certainly way too early to start talking about Jack White as one of the greatest guitarists of all time, I'm actually gonna go out on a limb here and say that a case could be made for him in a few years if he keeps up his current level of output. The albums, much as I like them, aren't really a good indication of what he's capable of. See the Stripes live if you get the chance: he's pretty goddamn impressive in concert.

And I'll echo a comment above: at least they had the sense to snub those tuneless hacks Satriani, Malmsteen and Vai.