If this build is for pvp (which I hope so, otherwise you fail at wow), you should take murder, and imp EA, skip lethality, and throw some points into a poison talent.
I have seen some math that shows lethality increases your damage less than 0.1%.
With shadowstep, if you are fighting a clothie, pre-med -> CS gets you to 5 cps in no time. You should put on a 5 point EA (IMO), with imp. EA and serrated blades, you get a clothies armor near zero. One of the (few) nice utilities of this build is, if premed is available, you get to 5 cps for very little energy. I am not big fan of KS, I feel like 5 combo points spent on EA or rupture give much better results.
EDIT: I liked 0 / 20 / 41 for a while, but "Not enough energy" is starting to make me sick. I am going to try 20 / 0 / 41 next.
Edited, Jan 11th 2008 10:44am by DrMayhem