Hmm, it's hard to see quite what's wrong here. It's a much more complicated layout than you need, which makes it harder to debug. One question: if you remove the AddThis code, does the problem go away? that seems the most likely cause to me.
The issue is somehow coming from the Share "button", which is an image set 110px down from the top of the span. I think IE6 is calculating the beginning point of the 110px from somewhere else... I really can't see why a span is there next to the menu, containing an image/button that's under the menu. Maybe the whole thing is supposed to really just be after and under the menu </ul>. Then the Share image/button doesn't need to be manually positioned so low.
Though that might also require having the main menu expand to its true height, instead of being truncated at 32px (a height is set on the li's, and are not really as large as the people-head images as you'd expect... this means the ul is only 32px tall and the box containing the ul is only 32px tall). Once the ul is full height the span could just com after it and sit under the menu instead of next to it.