Hmm, I do see that in IE6.
That AlphaImageLoader is going a bit psycho, adding lots of little square divs each with the arrow image in it. I'm not sure how to deal with that.
If only MS has spent more time getting the CSS right, instead of wasting its time on this proprietary rubbish!
If possible, I would get rid of that Image Loader thing, and maybe replace the png with a simple gif image and start from scratch with IE6. Sorry to be so vague.