I’ve not had time to look too deeply into your code, but I did notice that you’re using display:inline-block on the ul#nav li, IE6 doesn’t support this property on ul as far as I’m aware as they are naturally a block element.
I didn’t actually have time to check your site in IE (I have to boot up parallels to do that) so dunno what the problem was really – but I don’t see how a background colour would make the slightest difference. Perhaps it was a rendering blip at browsershots, it happens on your own browser occasionally, so there’s no reason to think it won’t happen to your browsershot once in a while – unlucky though if it does. Its perhaps a good idea to re-submit your browsershots request to check for such anomalies – yes this could take another 30 minutes lol.
This had me intrigued but it appears as though your colleague changed more than the background colour. display:inline-block was changed to display:inline so that’s why it works now!
I did wonder if his colleague was telling porkies (for non-UK readers that’s lies) but didn’t like to say. As I mentioned earlier, display:inline-block; isn’t supported in IE6 for Uls so needed to be removed. Whether display:inline; needs to be there I’m not sure – the list elements have been floated, so display:inline; might well be superfluous?