I've read through the forum and haven't seen anyone discussing why shadowbox hides dropdowns/<select> inputs on a page as the modal fires. You can try it right here on the main site. Scroll down until you see the dropdown for selecting the license type and click the Vimeo, Google Video, or Flash Video demos and watch as the dropdown disappears and reappears. Does the same thing on my implementation and some other sites that use it as well.
I can't figure out why it's doing that, but I'm going to have to hold off purchasing a commercial license until it's resolved. Unfortunately I cant have the 4-5 dropdowns on my page disappearing and reappearing because of a modal box... I'll keep checking on the forum to see if there are any solutions or if I'm the only one experiencing this.