It happened today with a 100-400 IS lens (which is kind of interesting since
I searched a few forums and found others reporting the same lens when their
20Ds locked up). My 20D has the latest firmware (it came with it).
My symptoms were pretty much identical to what has already been reported
here ... the camera focused but the shutter would not fire.
I tried another CF, another lens, power off and back on again ... no joy. I
did notice one thing that I didn't catch in my readings of earlier posts and
that is the camera reported ...