According to the mage guide post on the Nost forums, raid bosses everywhere have 145 resistance to Fire, Shadow, Frost, Arcane. 70 resistance to Nature, 15 resistance to Holy. Of course there are certain exceptions like Ragnaros having huge fire resist.
When curse of elements and curse of shadow are applied, resistance is reduced by exactly 75 - meaning bosses will still have 70 frost resistance. For mages, this is reduced to 60 because of the Arcane Subtlety talent reducing enemy resistance by 10.
Casting a binary spell (such as frostbolt) on a mob with 100 resistance will mean that it gets fully resisted 25% of the time. So if raid bosses have 60 resistance remaining, frostbolt is being fully resisted 0.6 * 25 = 15% of the time. Every 10 spell pen would reduce this by 2.5%, which equates to 2.5% more damage on average and makes spell pen very valuable.
The reason some people question whether spell pen is useful is because some people think that raid bosses have a certain amount of resistance that is impenetrable. However I can't seem to find a concensus on this. See this elitist jerks post for some discussion (
http://forums.elitistjerks.com/forums/t ... and-mages/). If the amount of 'impenetrable resistance' is >= 60 then obviously spell pen is worthless. I'd like to do some experimentation with this in some raids to find out. Or perhaps the mods will give some clues!