I have used a 350D for just over a year. it's ok except for the unusable LCD screen, the dark viewfinder and the noise in high ASA modes.
The 400D has a much improved LCD screen. All it needs to convince me to upgrade is a higher signal/noise ratio for 800/1600 ASA modes.
I photograph a lot of bird and animal images and many times in limited light. This morning, I came across a squirrel and took 80+ shots. Some of them turned out quite well but some could have been better if I'd used a higher ASA with a correspondingly higher shutter speed. 1/60th is a little slow for quick animals like squirrels. For many of the USA people reading this, you're probably thinking 'so what. it's just a squirrel!', but over here in Hong Kong, they're rare. I've only seen two squirrels in the 18 years that I've lived here.
I was shooting at 400ASA. Obviously, 800ASA would have been better giving me shutter speeds of around 1/100th but 800ASA on the 350D is rather noisy and grainy.
So if anyone has compared the 350D to the 400D in terms of noise at 800 and 1600, I've love to hear about it. Otherwise, I'll just have to put up with the limitations of the 350D until the upgrade for the 5D is released.
regards,
Gregory