I think I fit into this category, but I've shot more than just Canon and Nikon.
In three years, my system has been:
Pentax (K20D, K200D)
Canon (40D)
Nikon (D300)
Sony/Nikon (A850/D90)
Nikon (D700/D90)
Olympus/Nikon (E-5/D90)
Nikon (D700/D90)
and soon Canon again (1D3) and maybe a full jump to Canon, replacing my D90 with 60D.
There were various reasons for all my switches - I'm fickle, I'm curious, some things bug me and eventually cause me to switch.
For this latest switch, I'm trading a D700 setup for a very similar 1D3 setup, and I'm curious to see how the 1D3 handles (supposedly it doesn't have AF issues).
For my latest switch (still awaiting the 1D3), it's because of lenses only.
Nikon doesn't make a good 400 that I can afford, nor do they offer a cheap 70-200. The 400mm issue has bugged me for quite a while - I thought I had it licked with the Sony A850 + 70-400G combo, but eventually I just didn't like how the A850 handled faster action conditions. (70-400, while a bit slow focusing, was great).
So my plan is to ultimately get the 1D3, pray it has great AF, and set myself up with a 100-400L, 70-200 f/4 non-IS, and maybe keep the 28-70L I'm getting with the trade, though I might sell/trade that for a 24-105 or something.
My conundrum is the ultrawide, which kinda sucks for the 1.3x - I've read the Canon 10-22 works if you remove the back (from about 13mm or so), I'll have to find the thread on the 'net, but they had some pretty good suggestions.
To clarify, my D700 is outstanding and I'll miss it (though maybe not skin tones), but I don't have lenses from Nikon that I want (at least, not at the prices I want). As for focus points, they're pretty accurate in my experience. And low-light focusing is very good. On my lowly Tamron 28-75 f/2.8 and D700, it was able to lock (in AF-C) on a subject in my bathroom at ISO6400, 1/4 second. I literally couldn't see what it was, but the camera could.