I agree with all the above except I wouldn't go for the Olympus and can't say the Pentax attracts me too much. Why not the Olympus? Well, simpy because of two factors: I tried using one for an afternoon and truly hated the feel of it: It just didn't fit well in my hands at all. I do not have massive hands but they are not small and my fingers are fairly long (I have what is usually called piano player's hands and yes, i do play :o)) and the Olympus just felt awkward. Picture quality is ok, but that is going to be so on any of these, they are all decent cameras in that sense. The fact that the Olympus uses the 4/3rds system which does have considerably less availability than the others also gives you a factor worth considering.
I totally agree with what StephenO said about auto sensor cleaning, pointless if it just blows the dust around inside the camera. At some point, that dust has got to be removed, not simply shifted for it to return again just as easily as the first time it landed on your sensor.
I also don't like in-camera IS for two reasons: one is the problem if it goes wrong as mentioned, the other is that it corrects at the wrong end of the shake. IS lenses correct the shake where it is worst: in the passage of light through to the sensor. I have seen various comparison tests done and the IS lenses win every time.
Ok, so where do you go from there? Well, you don't need telling that Canon and Nikon cameras have superb build quality so that should be considered a big plus point. I can't see the point in getting the 350D if you can get the 400D just as easily. Don't get me wrong, if you were saving a significant amount, enough to leave you money you could spend on antoher lens then I would say go for the 350 as it is a superb camera. However, if you have the chance right now to buy a camera that is going to stay with you for the next few years, it does sort of make sense to go for the latest one now so that it keeps you going longer into the future. There are improvements in the 400D worth having for sure.
Whilst a lot of people don't do bigger than about A4, or at least 12" x 8", some do. I certainly do as I print a lot of pictures as posters at A3+ size. So is the extra resolution worth it? In terms of two pictures being put side by side, one of 8mp and one of 10mp, you'd need a darned good idea to notice much difference, even at A3+ size. However, the one thing that extra pixels gives you is extra room for cropping.
The Nikon D80 would probably go down as the best of the lot but you said you'd need to find a bit of extra cash for that so I don't see much point. You are not going to see any difference in picture quality over the 400D, though the build quality of the D80 would shade it.
So, the choice is yours, go for what feels right after you have tried them out in a shop. What feels great to me might feel awkward in your hands.
One thing is for certain, you won't be getting a rubbish camera whichever one you go for :o)
Happy Christmas... and open yours first so you can take pictures of the kids opening theirs... tell your missus I said so
Cheers,
Rob