excellent question: it's a topic that has been annoying us for a long time ... and I haven't found a correct answer.
BUT, the good news is:
nagios 3.x could be the answer with this:
nagios.sourceforge.net/docs/3_0/ ... ncy_checks
In total honesty: I haven't found time (or courage) to test it on our nagios3.x test server (we're running nagios 2.10)...
So, if you've got enough motivation AND you like the flashy/blinking icons of nagios 3.x, you can give it a try