David Mertz has an article up at ONLamp.com on Advanced OOP: Multimethods. Like before, this article is a rehash of this article from March (I won't get fooled again), but some things have been changed, so I figured I'd link to this for completeness' sake, and in case he's improved his explanation.
But more importantly, this gives me an opportunity to link to a(nother) great post from Dan Sugalski on Multimethod dispatch.
I found this article via LtU which also has some excellent discussion (on this, and in general) that's worth checking out.
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