Even though I am a "devoted" Nikon user, I'd go for the Canon 400D or a Sony Alpha A100 based on your circumstances. Also, all those cameras are great choices so it really boils down to how much you can spend.
I have price compared four 10-MP cameras with their kit lens, based from B&H photo (My favorite place to shop)
Sony Alpha A100: $66.90 Per megapixel
Canon 400D: $65.90 Per megapixel
Olympus E510: $58.50 Per megapxel (On-Sale!)
Nikon D40x: $61.90 Per megapixel
Considering they all come pretty darn close in price, now come lens choice, features, etc.
Hope this helps!
Assistant News Photojournalist
Nikon and Canon Systems
A variety of f/2.8 lenses from 10.5 to 400mm