Dagnabitall , the 5d is full-frame - I have been shooting DX for about 5 years now and the effective 300mm reach of my 200mm lenses has been perfect for the type of telephoto shooting that I predominantly do.
No big deal if you shoot canon as they have a nice looking 300mm f4 IS and 400mm f5.6 (no IS) in their range.
But I shoot Nikon - and would like to keep it that way, I plan to move up to FX as soon as the right camera becomes available (that's another post altogether though!)
Nikon's 300mm f4 is sans VR and the f2.8 VR is just too heavy to carry into the hills.
400mm is tough neighbourhood here in Nikonville if you are pecuniarily-challenged like moi.
Unless you have a sherpa-type dude and a spare $19,900 for the f2.8 or $13,900 for the incredible 200-400mm superzoom, you will just have to settle for the ancient, glacially slow and far from brilliant (at 400mm) 80-400mm zoom.
I notice that I am not the only one to feel this way nikon guru Thom Hogan has the following lenses on his 'waiting for nikon' list too.....
80-400mm f/4-5.6G AF-S VR
300mm f/4G AF-S VR
400mm f/5.6G AF-S VR
Oh well, we Nikon shooters like waiting......don't we (?)