My present backup is a 350D (Rebel XT), for the same reason as above: much lower cost, and good enough up to 12x18 prints. So the real question for me is how much better the 400D is than the 350D. I suspect the answer will be teetering right on the edge of worth it - darn marketing guys. 10 MP versus 8 MP only gives me 1/8th more print resolution: 216 ppi versus 192 ppi on a 12x18 print, but I'm thinking the cost will be well under $1k pretty shortly (like Christmas).
Although I call it a "spare" or "backup", there are times when I leave my 5D set up on a tripod waiting on a shot, or taking a series of shots - like sunsets - and will shoot handheld or on my lightweight tripod with the Rebel. So it's not like it just stays in the bag all the time. And though I have pretty large hands the Rebel XT grip doesn't bother me at all, though the viewfinder does after using the 5D. And the Rebel takes up very little room in the bag with the 17-85 lens that's almost always on it. Hmmm... I'm still going to wait and see some reviews to see what the high ISO performance is like. To me the Rebel XT is good up through 400, not so good at 800, and I'd be hard pressed to use 1600, even with Noise Ninja. On the 5D I don't hesitate at all to use any ISO through 1600 unless I'm pretty sure I'll be making a print larger than 12x18 from it - which is pretty unusual.
Once you've seen both viewfinders, I don't see how you'd ever consider a 400D a possible 5D killer. They're not in the same league.
Now if Canon were to come out with a 22mp sensor in a 5D body, I'd find a way to scrounge up whatever they want for it!