Theo you can have at this, as I stay away from the math portion of the game, but give this a read.
Brief synapsis:
5/5 lethality is anywhere from 1%-6% dps increase in PVE and 1%-3% increase in PVP(against 400resil targets).
Basically, point for point, Murder is going to always be better than putting a point in Lethality (and for PVP you don't really need it at all according to this).
http://wowriot.gameriot.com/blogs/WoW-Mechanics/Lethality-The-Truth
If you run a Sinister Strike build, Lethality will help you recieve bigger crits, but the total amount of extra damage given may not be worth the 5 point investment.
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Edited, Sep 13th 2008 8:46pm by MYteddy