I too, have had the unpleasant experience of breaking off the key
in the gas cap.  I've determined that this happens when moisture
has gotten into the lock and frozen.  There is a fairly easy fix:

1)  Remove the stub of the broken key from your keyring (makes it
easier to deal with)

2)  Stick the stub into the lock as far as you can get it to go.

3)  Using a heat gun (very carefully) or hairdryer, heat the cap
and key stub.  The stub will help conduct the heat into the lock
cylinder and melt the offending ice.

4)  When the key stub is almost too hot to touch, begin gently
working it back and forth.  Continue the heating/wiggling process
until you can unlock and remove the gas cap.

5)  On my syncro, I was able to release the lock cylinder from
the cap, then easily remove the broken part of the old key from
the lock.  Note, the cylinder was only removable from the cap
when it was in the unlocked position.

Do NOT drill.  If you have to forcefully remove the cap, I would
 attempt to break it off using a mallet.  If you drill you run
the risk of getting lots of small metal particles into the gas
tank and possibly an explosion if you create a spark inside the
neck of the fill spout.

Regards to all,
Jon B Kanas

IBM Global Services / IT Architecture
kanasjon@us.ibm.com
303-924-9049