This may be a little late.
On one ep of drawn 2gether they had a bunch of characters from tv ads.
On south park they did a story arch where characters from all forms of fiction live in "Imaganationland"
How can shows use such characters that may have some copyright attached to them?
One answer comes to mind is that parody is protected under the 1st admend.
Thank Larry Flint for that loophole.
-------------------------
"... when you're good and crazy, oooh, oooh, oooh, the sky is the limit."
"Because in this business reality can be *pretty* hard to come by."
- the Tick