I recall in a writing class being told that the root of all humor is conflict. I thought "no way, humor is good feeling, mirth and all that." But try to write something funny without conflict and you will see the truth of that statement.
Sometimes it seems all humor on the left is derived from two concepts; Bush can't read, Republicans are greedy. Now, neither of these concepts will get much of a chuckle from someone that doesn't share your political views and a possible confrontation will ensue. Conflict.
I find the knee-jerk reaction to Imus to be scary stuff. Censorship is ugly and intellectually stifling. If you think only the left can control who or what gets considered offensive, someday you will be in for a rude awakening. When all potential for hurt feelings is eliminated in the name of everyone feeling good, ironically what is left will be a dour, humorless world.