http://www.qdh.org.uk/wordpress/wp-content/uploads/2010/03/comboboxbug.png

The image above demonstrates a worst case for comboboxes near screen edges, where the user is presented with an empty box with arrows. This is not an acceptable initial state for opening the combo box.

The blog post here discusses this http://www.qdh.org.uk/wordpress/?p=358

Note: Pidgin has a fairly decent way of dealing with this now by moving the popup window to exist entirely above the combobox.

It should also be noted that when scrolling should occur, the box should not present empty space for elements to scroll in to, instead it should resize and move the window accordingly.