Hi Derek, with your existing investment in Canon lenses, I'd be tempted first of all to go for a Canon body.
Also it's crucial to work out if you are after something to complement or replace your existing system... If it's going to complement it and be used in different situations, like very light travel, then I'd be tempted to recommend the model with the biggest difference - the Oly E-410 - which can't be beaten for size and weight.
The Nikon D40x is also a wonderful camera, but I feel it's too similar to your existing kit to work as a complementary camera. You'll end up preferring one and not using the other. So feel free to change to Nikon if you like its system, but it would probably be as a replacement.
If you're fed up with swapping lenses, you may also be tempted by an 28-300mm (equivalent) superzoom. The obvious choice is the Nikkor 18-200mm VR, but now Panasonic has pencilled in November for the stabilised Leica 14-150mm for Four Thirds cameras, then that's something you may wish to consider.
So I'd say, Canon if you want to use your existing lenses. Nikon if you want to swap altogether. And Olympus if you want something nice and small for travel, although Panasonic's new L10 could also be a contender in this respect...
See: http://www.cameralabs.com/reviews/Panas ... x_DMC-L10/