After upgrading to NetBSD-current from 3.0_BETA a few weeks ago, I noticed entries like the following in my log files:
Feb 4 16:47:14 tesla rwhod[19995]: whod.tesla: Permission denied
Feb 4 16:48:10 tesla rwhod[19995]: whod.sign: Permission denied
Yes, for some inexplicable reason I do still run rwhod(8)…

Checking /var/rwho, I noticed that its contents were owned by root:wheel and therefore no longer writeable by rwhod, which now runs as _rwhod:_rwhod. A quick “chown _rwhod:_rwhod /var/rwho/whod.*” solved the problem.

I did some hacking on postinstall(1) and filed problem report 32731 which contains a patch that changes the permissions after an upgrade.