Why do form elements show through layers?
This is rendering quirk found in both Internet Explorer and Opera on Windows, and unfortunately there’s nothing that can be done about it short of redesigning the page.
Though Opera allows all form elements to “shine through”, this only happens with select menus on Internet Explorer.
This happens because those browsers display the Windows’ native form elements as opposed to creating them from scratch as browsers like Netscape 6.x/Mozilla do.
Similar problems occur with plug-ins and layers in most new browsers.
The DHTML Lab has a more detailed article explaining the situation. Thomas Brattli has a JavaScript solution to the problem as it relates to his CoolMenus script (the fix essentially hides form elements appearing under layers and can be tweaked to work with your own page).