I'm using nrpe with xinetd without any major problems.
1.) On the remote machine, ou have to make sure to have any neccesary libraries needed for the plugin to work.
2.) include the port number in /etc/services
3.) set up a inetd entry which starts nrpe
4.) edit nrpe.cfg to reflect your plugin needs.
I think problems with the whole thing can only be solved with tools like strace (linu:evil: or truss (solaris).
Or maybe you'll see some additional infos when looking at the network traffic with snoop or tcpdump.
For example, i had a problem that for some reason, the shell which is invoked on the remote machine tries to read root's .bashrc and has no success. So it says "/root/.bashrc : permission denied". And thats all the plugin ever said since nagios likes the first line of output only ...
So i had to allow the nagios user to read roots .bashrc.
Have you ever tried to run the plugin on the remote machine? which output do you get?