I believe that as the "Magic Resistance" spell is technically considered a defensive spell, the dragon AI does not kick off.
If I recall from my wanderings on the Black Isle boards, the designers are making lots of changes to dragon AI. I believe that one of these changes is to have them go hostile upon any sort of spell casting. Someone correct me if you know better...but I'm fairly certain on this...
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Now where did I leave that doughnut?!