Since IE7 has come out, a lot of people have noticed that Suckerfish dropdown menus are allergic to it. The principal problem is that once they've dropped down, they refuse to scroll back up again. This affects a lot of Joomla sites using Suckerfish variations and also the Extended Menu module.
The solution is thankfully, simple. If your list menu ID is called "topmenu", you only have to add the following lines of code to your css:
#topmenu li:hover, #topmenu li.hover {
position:static;
}
Thanks to Built from Source for the original tip and to Sam from Site Coffee for Joomla implementation.