I have had a 5D for a year and I have been MASSIVELY disappointed. I have used Canon film SLRS for the last 10 years and at least 90% of my shots were sharp and quality. With my 5D I would say 90% are out of focus, soft or blurred. I thoroughly hate it - e.g.: it's a bright day and I take a pic of two smiling friends. The LCD is so pants it looks okay. But when I get home it is so blurred to be unusable (though the rubbish LCD makes everything look soft, and as I say, most shots are). This is so common with this camera. I am tempted to send the camera to be looked at by Canon because there is something very wrong with it. Medium format quality my backside.
One of the problems I have found is that the mirror slap is massive and is probably exacerbating the blurring problem. The weight of the camera with a 24-105mm lens doesn't help too. But I have taken shots on a tripod and the focus is wrong or the corners are soft or the shot is simply blurred.
So I am looking for a lighter, smaller DSLR camera. The Canon 40D is only marginally lighter than the 5D which is not helpful and the 450 may be too much of a downgrade, though at this stage I'll take anything that gives me something resembling sharpness. The Nikon D300 is also a bit on the heavy side.
I have a number of Canon lenses, but I am willing to trade them in for a different system if need be.
The Olympus 510 appeals with one caveat - the small viewfinder. Having a large viewfinder really helps as I wear glasses and I like to manually focus at times.
Nikon's D60 is another option, I fear the D300 may be on the heavy side, though I did find the shutter to be quicker and lighter than the 5Ds.