When I scroll down on your site and background image goes up, there is some gray gradient or something like that and it isn't looking smoothly. Maybe it's because of my laptop screen (it's not too good).
So I suggest you to use something like on twitter, to use fixed background, try scrolling there. I've tested that code using Firebug and as I've said it looks better. Here is a screenshoot of your site with fixed background, but you need to see that in action .