Created a Swing application with the NetBeans gui editor. Worked fine initially. Some time later the message “package org.jdesktop.application does not exist” showed up and I couldn’t fix it.
Solution found here: http://forums.netbeans.org/post-21150.html
Basically, deleting the cache directory fixed things up.
Windows: C:\Documents and Settings\your_user_name\.netbeans/6.5/var/cache