I noticed on my rawhide VM that rpc.idmapd was almost never getting started at boot time. The problem seems to be that systemd was trying to start it very early in the boot process, before any of the other nfs services were started, and sunrpc.ko was not plugged in yet. When this happens, rpc_pipefs has not been registered as a filesystem yet so the mount attempt of it fails. I was able to fix this by adding /etc/modprobe.d/sunrpc.conf with the following contents: alias rpc_pipefs sunrpc ...with that, the nfs-idmap service seems to start correctly. I suggest that we ship a similar file as part of nfs-utils for now. There's an upstream patch to add this alias internally to the sunrpc module, that Bruce is going to push for 3.2. That should eventually make this file unnecessary but it should be harmless even with that in place.
cc'ing JCM... John, does this look like the right thing to do for now?
The "alias rpc_pipefs sunrpc" now exists in /etc/modprobe.d/dist.conf