I don't see it on Firefox 3 on my Mac. Is the background scrolling with the posts, or is it fixed (that's what I see)? I've seen IE have problems with picture backgrounds, but that was IE 5 and IE 6. I don't remember seeing it with IE 7.
Okay, I hopped on my wife's Windows XP box and I see what you mean. On both IE and FF3, I think it is because the background is fixed and the rest of the page needs to be re-drawn on top of the background. I don't know if it's the integrated MB graphics chip on my wife's computer or what, but it takes a tenth of a second or so to start scrolling. My Mac doesn't have an issue with this, even with Folding in the background.