I spoke to a GM tonight (Koenigspiel) about this, and apparently it's a known issue: If a chocobo is set to resting, even if its care plan is then changed, it can fall asleep and be impossible to wake. S/E is aware of the issue and they can't do anything but apologize. For now, I mean. I was told they were working on fixing it but he was pretty careful not to give me any expectations as to what would be done and when. [edit: He did, however, apologize several times.]
My chocobo executed the Taking a Walk care plan, but remained asleep and impossible to wake/feed, and its energy bar is all gone >.>
Edit for clarity's sake: The GM didn't know what caused it or how to avoid it, so it might be safer to avoid the 'Rest' plan completely until they have it worked out. Once it's stuck sleeping they had no advice on how to wake it or what to do. The chocobo isn't intended to get stuck sleeping while executing other care plans.
To repeat: When the bug occurs, the chocobo will continue to execute other care plans (listen to music, take a walk, etc) but remain stuck in 'Sleep' mode and cannot be fed. Some people have reported their chocos recovering spontaneously, others have been stuck for days.
Edited, Aug 29th 2006 at 8:44am EDT by Aloof
[further edit: Title changed to better communicate the warning]
Edited, Aug 29th 2006 at 2:51pm EDT by Aloof