As a daily user of APS-H bodies since the original 1D, I'm going to badly miss the reach when I shoot with the 300 f/2.8is on the 1DMk4 when I update to 1DX (when they actually ship!). 300mm x 1.3 = 390mm on APS-H.
I figured a 400 f/2.8 will be an inevitable purchase, but the 400 f/4DO has caught my attention. The 400 f/2.8is is a heavy sucker of a thing...the DO is a featherweight by comparison. The lighter, smaller 400 f/2.8 MkII would be brilliant but I'm reluctant to drop close to $12K on one if I don't really have to.
Canon EF 400mm f/2.8 L IS USM Lens (163 x 349mm) (5,370g)
Canon EF 400mm f/4 DO IS USM Lens (128 x 233mm) (1,940g)
Canon EF 400mm f/2.8 L IS II USM Lens (163 x 343mm) (3,850g)
Canon EF 300mm f/2.8 L IS USM Lens (128 x 252mm) (2,550g)
If the DO has the IQ and the AF performance, it could be a great match to the 1DX with its improved iso performance and claimed big advances in AF. The lowest cost alternative would be the drop a x1.4 converter behind my current f/2.8 300is.
I'm not completely cost driven, I just need to maintain the reach I currently have with APS-H when I move to the FF 1DX in a high performance, sensibly cost effective setup.