I just got my Raspberry PI and installed Nagios on it. Runs well as long as it doesn't have many services on it. It can hold with no problem approx 50 hosts with 300 services (normal checks – NOT PERL!)
I did not used the official Debian package (as I wanted the latest version) so I have compiled the packet myself.
Had my eyes opened once I have started to use Debian as until now I have used only Open SuSE.
I have written two words about the install: http://i-security.ro/linux/raspberry-pi ... monitoring