I recently had trouble with IDEA 5.1 being unable to contact the licensing server to get the site license. This problem might be fixed with later releases of IDEA, but if not, here goes.
The problem was that I had installed ActiveSync in order to connect to my Windows Mobile device. ActiveSync created a fake network device and treated the PDA as a networked computer. IDEA, when trying to acquire a license, did not differentiate between the fake device and the real device, so it queried the PDA for the license data and failed (of course).
The fix was to disable the fake device until IDEA had successfully loaded, then re-enable the device to resume communications.