Putting aside 5DmkII, I have noticed ever since Canon introduced DigicIV processor, it seems that their pictures have more noise to them.
Why I mentioned this because I used to have their compact IXUS860IS which uses DigicIII processor which really impressed me with the low noise and fantastic video quality (although VGA Only).
When IXUS870IS was out (with DigicIV processor), I thought it should be better and sold my 860IS to buy it.
However, I was quite disappointed with it's noise level and video quality that I sold it off few weeks back when I got my LX3.
Probably it's just me, but this may be one of the reason why 450d is performing a bit better than 500d in noise level.
Hopefully Canon should improve again in upcoming DigicV in 60d which "suspiciously" retained the 15MP...