Image quality and features: Canon 40D
What I'd choose: Nikon D80.
I'm sorry, but how a camera feels in my hand is important. I don't think people realize how little difference in photo quality there is between a Pentax K100D, Nikon D40x, Canon 40D, Sony A700, or a Nikon D300. Very little. Very
very little.
Why do you take photos? To capture the moment, or to document something, right? All of these cameras can do this, and the images are almost all of equal quality. OK, so image quality is around equal, and therefore, a non-issue. Assuming you're a casual user and just want some casual,arty, or photojournalist style shots, the MOST important thing is how the cameras feel ergonomically.
If your photographic needs are greater than the above, the lenses, autofocus (AF) system, and flash system ARE a real issue. What do you want to shoot?
If you want to do "birding" and take photos of birds in flight, I've heard that Canon has slightly better lenses for the job (and more options) while Nikon has the camera because of the AF on their mid to high end cameras. Nikon still has the good lenses, though. The variety isn't quite as great as Nikon's, and some of them need updates to add VR and AFS.
Sports? It's between Nikon and Canon. You can add Sony as the 3rd option. In the past, Canon had the benefit of better high ISO noise control, which allows professional sports shooters to use a higher ISO and a faster shutter speed. Very important for pro sports, but not to me. Currently, it's either a tie, or Nikon has the lead. I don't think anyone can say Nikon is behind right now.
I have also seen some excellent Sony shots. Sony have the lenses to do the job as well, and the A700 has the AF speed to keep up with Canon and Nikon. I wouldn't get a Pentax or Olympus for this type of job.
Portraiture? I'd say Nikon. Better flash system than the rest.
Macro? Probably a tie between numerous brands. If you want to get a macro lens at around 100 mm, then everyone has that option.
You want image stabilization in every lens because you have shaky hands. Pentax K10D and Sony A700. The A700 is suppose to be a fantastic, fantastic camera that's equal to the 40D and D300 in image quality.
You want ruggedness....the toughest camera out there. Probably the Olympus E-3. Their lenses are also the most prepared for bad weather. Heavy rain, buildings tumbling around you, I'd probably put my trust in an E-3.
Ergonomics? In my opinion, Nikon feels right. Sony is also very nice and feels natural. The Pentax body is supposed to be good, but from my limited experience (around 5 minutes in Japan), the K10D took me a few seconds to figure out. Maybe it's because I'm used to Nikon.
I found it much harder to get used to the Canon 40D. Sorry, I just can't use it. Same with the 5D that I've held (only twice). I thought the Canon 400D/Rebel XTi was hard to get used to....until I had to use the 40D.
Weather sealing isn't an issue unless you like to shoot in sandstorms, pouring rain, or flying sleet. I can't think of many reasons I would want to take my $3000 system out in those, weather sealed or not.
When you're shooting on a beach, on a boat, or even on warm, damp, humid days.