It depends on what you are doing, but if it is mostly snmp data collection and icmp/tcp application monitoring, OpenNMS will probably do it out of the box with autodiscovery and no client setup. If you have lots of custom nagios client code, you'll probably have to twiddle some ugly XML config files to get that data collected. The mail list support is fairly good if you have problems.
Les, Compared to Nagios, how difficult was it to get OpenNMS running in your environment? I found Nagios trivial but have never really rolled my sleeves up with OpenNMS, I have just sort of kicked the tires over the years...
I have to say the WMI capability of Zenoss is a real plus for me and the documentation for Zenoss looks way better than OpenNMS which even they admit on the wiki isn't very good:)
jlc