Matt mentioned Shepard Fairey as an example of someone who "pay homage" to previous artists in his work. Take a look at some of the examples on this page and decide whether it's paying homage or outright theft.
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In some instances a good case can be made for fair use, but in others Fairey just sticks his trademark on a previous artwork and sells it as his own. It's also of note that Fairey has threatened to sue others who did the same thing with his work.
I'm reminded of Keith's rants on Podshow appropriating KATG as part of their "network" of podcasts. I'm not sure if Podshow just redistributed them or inserted their own ads and, like Fairey, used others work for financial gain.