Hi, Fred,
Nikon has two lines of their pro cameras, too -- the D3 and the D3x, one is high speed lower resolution, the other is very high resolution at lower speed. Just like the Canon 1D vs 1Ds series.
The 1D series is targeted at photojournalists and sports photogs, and has been since the first 1D model (of which I still own two.) The 1Ds series is targeted at commercial and studio photogs, just like the D3x. (Note that the 1D series has an APS-H sensor, which is a 1.3x focal length conversion, not the 1.6x of the Canon APS cameras like the 7D, 40D, etc.)
In the past this was a much larger difference -- as computing power has improved, the "high speed" 1D series now has as many pixels in the 5th generation body as the 1Ds camera had in its second generation (1D Mark IV versus 1Ds Mark II, both at ~16 megapixels.) At some point, the 1D series body has enough resolution for pretty much any assignment, and I think we've reached that point.
As an aside, let me say that I agree with Jeff and wish Canon had gone with a full frame solution for the new 1D series camera. While I have been shooting the 1D series since it came out, I'd much prefer a full frame sensor at this point, but I need the shooting speed.