You may want to check with Magnum, but from his screenies and videos of the Dirt3 Single Player race/timetrial mode in the screenshot forum, it looks like Dirt3 comes with 85%+ of the cars unlocked without having to do the career (even the Group B classic cars)... I haven't had much time to play yet and I am focusing on the career mode, but that is what it seemed to me as well from looking in the "Garage" where it list cars.
So from what I could tell it looks like the "unlock the cars" situation is minimal compared to Shift2/GT5/Forza3 etc...
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--John
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