I first tried removing the memory card and restarting. Failed.
I tried removing the SIM card and restarting. Failed.
You can't re-install the firmware because it doesn't get into any of the data modes. I wasn't going to send my N82 back to Europe for servicing and be without it for a long time. I was cringing at what it would cost at an independent store.
Thankfully Nokia added a fail safe. You can wipe out the phone memory (lose all your contacts if you have it set to phone memory, notes, etc.) and restore it back to the last firmware on the phone by (a method commonly known as the 3 finger salute):
Hold the green call button, the "3" key, and "*" key all at the same time while starting up the phone.
If you have 5800 (firmware V. 20.0.012), hold the green call button, the red hang up button, and the camera button all at the same time while staring up the phone.
If you have N97, hold the "Caps" key, "Space bar" key, "Backspace" key, and "Power" button all at the same time while staring up the phone.
The phone will slightly flicker and prompt you for your country. Now my N82 is back to a factory clean state.
*UpdateYou can also do this on S60 5th edition like the 5800 (firmware V.20.0.012 or higher): hold the green call button, red end call button, and camera key all at the same time while starting up the phone.
Note: I'd only use this as a last resort.
Note: This doesn't save you from a botched firmware upgrade using Nokia Software Updater.
Note: Be careful with your phone.
Note: Although I did this with a N82, I'm sure it'll work on other S60 phones like the N95.