I just noticed this little gem in the LUA section of the 3.3 patch notes:
You can now query for a list of completed quests with “QueryQuestsCompleted()” then wait for the “QUEST_QUERY_COMPLETE” event, and call “GetQuestsCompleted( <luatable> )”.
I believe this means we can expect to see mods shortly after the 3.3 patch is released that will show your question completion history. Pretty cool IMHO.