Oracle Application Express Release 4.2.1.00.08 was released on December 14, 2012. A good oportunity to take a look behind the scenes and see what actually changed.
If you are coming from APEX 4.0 (or older) read the Posts about the differences in Oracle APEX 4.1 and the Patchset 4.1.1 and the changed things in APEX 4.2 at first.