Note:IE6 and IE7incorrectly apply z-index: 0 to all positioned elements. The 2.1 spec says that positioned elements that are z-index:auto should remain z-index:auto unless explicitly changed
The problem is that you have some positioned elements after your dropdown and IE6/7 give them a higher stacking order.
It should work if you set a z-index on you nav ul along with your RP.