I made the opposite switch (Canon to D3x), and I'm very happy with both the ergonomics and the IQ of the Nikon. At least between the 1Ds mkII (older) and the D3x, the resolution jump is more than expected, because of Nikon's superb AA filtration. The f4 lenses are a real Canon advantage, but remember that the difference in weight between a D700 and a 1Ds mk III is AT LEAST the difference between a 24-105 f4 and a 24-70 f2.8 (and the Nikkor 24-70 f2.8 is a significantly sharper lens than the Canon 24-105 f4, although without the reach). The 70-200 f4 is a VERY sharp Canon lens, and my message to Nikon on that one is: COPY IT ALREADY!!! Longer Canon lenses tend to be better than the Nikkors (better selection, some lenses are sharper), while Nikkor wides are often better than Canon.
In terms of the 5D mkII in particular (the 1Ds mk III is MUCH closer, and superior to the D700 in some major areas), remember that you're taking a big downgrade from the D700 in build quality, metering and focus. The 5D mkII has a lot in common (other than the sensor) with a nearly 5 year old EOS 20D, because the original 5D was built on a modified 20D body, and the mk II is mostly a sensor transplant (plus a new screen and some processing upgrades on the digital side, but not much to camera features). In particular, the 20D focusing and metering systems are pretty much intact. The D700 has brand-new, professional D3 heritage instead of 4+ year old cheaper camera.
I also don't personally like Canon ergonomics nearly as much as Nikon - the "modern Nikon interface" we first saw on the F5 is just a great way to control a camera. If I were you, I'd stick with your D700, and hope for some f4 Nikkors (hint, hint, Nikon). Both systems have some great camera bodies and lenses, while neither one has everything you might want. Nikon has a habit of "getting stuck" at one resolution for a long time - they released four years worth of variations on a 6 mp theme until they doubled that with the D2x, then they did the same at 12mp until the D3x came out, while Canon tends to upgrade sensors much more frequently, but not offer their best metering, focus or build quality outside of the
EOS-1 line.
-Dan