I had the same thing on my car for the longest time after I got it... Strangely, cursing at it didn't free it, regardless of volume - I even tried a couple different languages.
On mine, although the key turned in the lock, and the inside knob moved slightly, it didn't have that positive "click" when locking or unlocking. I finally decided one day that I was going to open that door if it killed me... So here's what I did:
First I removed the door panel on the opposite side door which DID open. The reason for this was so I could see exactly how the rods/levers operated on a working door.
Once I was comfortable that I understood the mechanism, I went to work on the stuck door. I rolled the window down all the way - and started by first liberally dousing every contact point with some WD40. I then used a variety of things to move the various pivots around. Eventually, I found the offending lever - as I recall, it had a sort of paddle shape to it... it was stuck halfway and wouldn't budge. I hosed the pivot point with WD40 to loosen up the grime, snaked a long 3/8" extension through the window opening and nudged it back into position. The door opened, but the lever would still get stuck. It wasn't till after THOUROUGHLY removing all the years of dirt and grime, and re-greasing the pivots that it operated properly.
My guess is that on the older cars, more dirt gets past the weatherstripping than usual... this gums up the works, and the least-used doors are the first to get stuck.