This is a well-known bug with IE6 with trying to absolutely position divs on top of select controls.
There are workarounds involving iframe shims, but there is no good answer. The iframe shim answer is to place an iframe underneath whatever div you're trying to place above a select, with the iframe being the same size. Unfortunately, this is a huge pain in the ass.
I suggest using a javascript menu system which already uses iframe shims, such as YUI menus (and I assume jQuery).
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