Well, I'm not surprised. 'Bad' has been used as a (slang) synonymy for good, interesting, fantastic, etc. for a long time here in the US (I'd unscientifically guess it started in the 80's), so I can easily believe that 'wicked' would follow.
Give it a bit more time, and I wouldn't be surprised to see that usage of wicked=good pop up in AmE as well, if it hasn't already.
Actually, kind of an interesting question to see if wicked=good will take root in AmE, since there's already a usage of wicked=very, especially since that usage is regional.