Just solved this (didn't see anyone else with the same problem in searching the forum), and figured it may help out other people:
At lunch today, went out to my car, put the key in the ignition and attempted to turn it to start. Key went in just fine, started to turn, but got stuck as it approached the 'off' position. Car is a manual, and the key turned far enough to release the shifter from reverse. Tried turning the key several times and each time, as it approached the 'off' position, it felt like, and sounded like, whatever was turning was hitting something metal. (sort of like if you were trying to walk through a doorway with a piece of metal that was too wide and it kept hitting the frame - back up, try to walk through, hit the frame.)
Following the suggestions from other threads talking about lock cylinders, I disconnected the battery and liberally sprayed WD40 into the cylinder, working it in with the key (moving the key up and down, and turning it as far as it would go, multiple times quickly.)
The difference between my problem and those mentioned in other threads was that the center part that 'pops up' when you remove the key, (and gets pushed down as you turn it) was 'popping up' just fine - didn't seem to be sticking at all.
Finally I got frustrated and started to wiggle the key rather aggressively as I turned it. Still nothing.
I pulled the key most of the way out, so that only about 1/8th of an inch was sticking in to the cylinder, only in the part that 'pops' up, and jiggled furiously on just that little top portion. This last part was enough to do whatever needed to be done, and the ignition works and turns flawlessly now.
Not sure, but I think the combination of the WD40 and wiggling that top little portion solved the problem.
Again, just posting in case someone else has this issue.