The D3 is aimed squarely at the sports and PJ market; the combination of high ISO sensitivity and a full-frame sensor at 9FPS will make for a great workhorse camera.
The 1D Mk III still puts more pixels on target when you use a given lens owing to its "crop" factor - if the D3 sported a 1.3x 'crop', it would have 7.1MP in resolution assuming the same photosite size/density.
That said, the 1D Mk III currently suffers from inexcusably bad AF performance in bright, warm conditions that many outdoor sports take place in. Whether this can be remedied by a firmware update remains to be seen... the last two updates only fixed very minor issues related to the AF module but didn't solve the major ones.
IF Nikon's AF performance on the D3 eclipses that of the Canon camera, you can bet that there'll be a lot of pro photographers considering the benefits of getting a D3 setup. Some of the smarter ones that took the long view may still have their older Nikkor lenses.
So - with the D3 hitting the shelves in three months, Canon need to get the 1D Mk III problems ironed out inside that time... or else the whole sports/PJ market might undergo a sea change.
Oh, and for what it's worth: yes - I'm considering a pair of D3's to replace my D2X bodies - but not for a while yet. I'm going to wait about six months and probably rent one first to try it out.
What I
will be doing, though, is picking up a D300 for the Mrs.
At least that way my DX glass will still be valid.