I Just encountered the same problem today when I pasted the opacity keyframes from one layer to the layer below it. Both layers are 720×486 JPEGS in a rather long slideshow. From that layer on, any keyframe I place, is locked and unmovable. The odd thing to me is that I am still able to move all the keyframes that existed before the copy and paste occurred. I have tried turning off the Live Update switch, but the problem remains.
Luke Bishop
2 x 2.66 Ghz 6-Core Intel Xeon 24 GB RAM OSX 10.6.6