Bug #87382 bit me:
# mount -t nfs4 sheep:/nfs4 /mnt/nfs4
...although rpc.idmapd was running (on the client). Now I have to:
Warning: rpc.idmapd appears not to be running.
All uids will be mapped to the nobody uid
/bin/pidof rpc.idmapd > /var/run/rpc.idmapd.pid
But the real question is: why does mount(1) bother for pidfiles, rather than checking for functionality?