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On Windows, a long <option> will be fully displayed, potentially causing it to go off the edge of the screen (I had this happen in Drosera's symbol popup). On OS X, the width of the menu is restricted which is definitely better than the current behavior.
For what it's worth, the behavior of other browsers is as follows:
IE 7: Restricts the pop down menu to the width of its parent <select>.
Firefox 2 & 3 (Mac & Win): Behave like us and fall off the edge of the screen.
Opera (Mac & Win): Restrict the width of the menu and scroll the content like a marquee.