The best example of a milestone camera is Canon’s 5D. Even on the day that it came out nothing about the 5D was really novel or redefined the absolute state of the art, but it put then-prohibitive quality in the hands of devoted middle-class photographers. This isn’t to say that the 5D is or ever was “affordable”, but many will find money for their passions (cars, motorcycles, boats, golf, whatever… cameras) if the golden opportunity presents itself. For many serious amateur photographers the 5D was that opportunity and it created a class of cameras, the “5D-class”, which to this day comprises of the 5D alone (but this is set to change soon).
As well not all milestones cameras are great, but it helps a lot. The most recent example of a milestone camera that stirs only mild excitement is Sigma’s DP1. Independent reviews that I am inclined to trust (i.e. from people I trust) paint the DP1 as a brilliant idea, an engineering success (particularly the lens) which then got wrapped into a cheap gizmo. It would be sad if no-one ever refers to the “DP1-class” cameras in the future, because that would mean the brilliant idea died with the DP1 (not that the DP1 is dead yet, it was just born).
Back to our topic of the day then: is the D3 a milestone camera? Yes. I'm sure of it.
In more ways than one it marks the end of Nikon’s first digital era (started by the D1), and the beginning of its second. The D3 carries the flag for a serious advance on image quality, restores the best of the film heritage and does away with old baggage at the same time. It defies classification by referring to every professional camera out there at once, while also maintaining a safe distance in comparison.