I tell ya' Hawk, the comma implies a space or pause and frankly I don't pause when I say "Listen up Motherfucker." I think my real mistake there was in writing Mother and Fucker as two words when in reality everyone knows it's a stand alone word all by itself. If I were wanting to really accent the thing for max emphasis then it needs to be:

"Hey!!! Yo!!! Motherfucker!!! Listen Up!!!"

Also, to address Ebbie's comment about applying to male and female, "Motherfucker" can work, but I think to be absolutely politically correct, a better term that can apply equally to either sex would be "Cocksucker."

Spaw