Since simple category is an adapted version of css flyout, it should be able to fly as well:
http://demo.rubikintegration.com/zencart/

Certain changes needed to be made tho: the level starts from 0 instead of 1 (blah, my coding habit), and the class names are not the same.

Brief tut:
https://svn.rubikintegration.com/zen...t_menucode.txt

Special thanks to the Jade True, creator of css flyout tree for the code.