First of all thanks for both authors of "Combobox" (http://oracleapexideas.blogspot.com.es/2012/06/combobox-plugin.html) and "jQuery menu" (http://blog.theapexfreelancer.com/2011/02/jquery-menu-apex-plugin) plugins! Both are great plugins!
I have a conflict when I insert both plugins at the same page. The problem is, jQuery menu cease to work.
I've recreated the situation under apex.oracle.com, the dropdown menu does not work anymore after inserting combobox plugin: