Sometimes a system administrator needs to get around a few rules that are in place for good (or not) reasons. One example is when networks have ICMP turned off (or even just a portion of it). With ICMP off it can be difficult to configure tools like Nagios for simple up and down checks. In this text getting around the no ICMP problem and a script to handle it for Nagios.
Read the full thing at http://systhread.net/texts/2010q3nmap_ping.php
I wrote a similar check a couple of months back. This one is much better :)