i went canon in your situation. I preffer the menu on them, they feel nicer put together (not everyone will agree with that) However the nikon has a beter hand grip, the 400d has a v small one.
Main thing that swung it for me was the choice of long lenses, nikon are well catered for by sigma, and have a few of their own tele photos, however canon have a massive range, and sigma cater well for them as well.
Dont regrett the 400d at all apart from not getting the 40d.
BTW the d40x or d60 is more of a nikon equivilent.
Also bare in mind the 450d coming out march time, and the next cash back offer soon to arrive.
Also look at the sony alpha a200, has built in IS, much nicer to hold camera than nikon or canon. Just a bit more expensive and a much smaller range of lenses (at the moment, getting better and sigma now provide minota (sp?) fit lenses)