I had an 89 F250 with the 'key wont even turn' problem. The solenoids used for the power locks will freeze up solid. I was a cheap bastard and just snipped the rod that goes between the solenoid and the lock and used the key from then on.
However my '00 F350 has the "weak solenoids" issue - takes 2-4 button pushes to get the power locks to lock or unlock - which will eventually lead to them not working at all. There's a fix for it (besides just replacing the power lock assemblies) I just haven't done it yet.