If you ever wanted to remote debug the PL/SQL code which is called during execution of an Oracle APEX application, then have a look at the May/June issue of the Oracle Magazine. David Peake wrote an article titled The Power of Two on how to use SQL Developer to do that. On the German Oracle APEX Community Web-Site you will find a similar how-to in German.
Update: In Oracle APEX 3.1.1 there will be a new debug mode called REMOTE to enable remove debugging in Oracle APEX without having to modify the application. See the related posting on the OTN Forum.