Of course, one must also ask the question you *really* want a dropdown combo when have more than 250 items. Once you've got a large number of items, some of the normal behaviors of the dropdown combo -- such as disappearing if you click outside of the dropdown list (or how about PgUp/PgDn -- something that's probably natural to use especially with a large number of items), or typically having a fixed size (i.e. user can't grow/shrink the list) might prove to be an annoyance to users.