Slur vs. Humor

I'm hard pressed to identify the true difference between a racial slur and race-related humor. Don't me wrong, I can tell when I hear it--if you asked me to define the difference, I'd have some trouble.

I suppose it all lies in the intent of the statement.
A black man going, "What's up ma nigga?" is acceptable.
A white boy in the 'hood with a du-rag saying the same thing, may or may not be accepted.
This example is less about humor than acceptability...but still, the point remains.

This cracked me up today, but I think parts of it offended me as well--so I'm trying to decide what category I'd put it in...slur or humor?
You Know You're Indian When...