Yes, I have been back and forth many times (3 times exactly) most recently from D700 to 5d2. I got a killer used deal on the D700 and sold for nearly $1,000 profit to fund 5d2 and my glass. The only premium glass I own for my 5d2 is the 35L...unless you consider the 17-40L to be premium. HAHA. Just kidding its merely mid-range at best.
As I've beaten to death on this site, I do have the D800 on order. I still have some decent AF Nikon primes (50/1.8 and 85/1.8 AF-Ds) as well as the Tokina 28-70/2.6-2.8. When my camera ships, I am going to RENT a 16-35VR and Nikon to Canon adapters from Lensrentals.com to test both cameras extensively. I will also shoot my Canon 17-40L on the 5d2 against the Nikon 16-35VR on the D800 body. Stopped down to f/8-11 this will tell me corner to corner sharpness differences at comparable FL (18mm,21mm,24mm,28,35mm). Lastly, I will test the Canon 50/1.8 mk. 1 vs. the Nikon 50/1.8 AF-D,each on their respective bodies. After running through lightroom 4, and many tweaks, and absolutely ZERO DOWNSAMPLING, this will tell me real-world results.
I expect the D800 and 5d2 to be very close at 3200 and 6400 at Full Resolution. I am not buying the D800 to use at 21MP, or 18MP, or 12MP. I want to see its ISO performance at 36MP. Can't wait to test these out!!