There's a scary amount of misinformation in this thread, particulary stuff like :
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As far as the limited testing that people have done with spell pen + mages the "agreed upon" conclusion was that COE/COS removes the resists, not that mobs don't have resists. As far as damage gains on a mob with resistance I believe it's pretty cheap item budget wise.
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Please use the search function. The *only* thing that came out of boss parsing was that there are no resists on any boss in TBC and original WoW, apart from a vcouple of oddities in Mc / BWl, and that the partial resists you get due to your level difference cannot be modified by any stat.
Spell penetration is a pure Pvp statistic, and should be completely ignored by anyone who doesn't PvP.