Recently I browse the web a lot to look for discussions about drupal monitoring, how people are solving it today and what the pros and cons of existings solutions are. Here is an interesting list with some decent discussions:

http://www.chapterthree.com/blog/josh_koenig/enterprise_drupal_needs_mon...
Interesting thoughts of JOSH KOENIG (Chapter 3). He thinks that Drupal can only play a significant role in enterprise when extensive monitoring is "built in".

http://drupal.stackexchange.com/questions/2627/monitoring-strategies-for...
List of existing tools including non Drupal related solutions.

http://it.toolbox.com/blogs/szymon/drupal-performance-monitoring-with-ne...
Interesting post about how NewRelic helped to identify performance bottlenecks.

http://benbuckman.net/tech/11/04/monitoring-drupal-sites-munin
Ben Buckman explains how to setup Munin in order to monitor Drupal.

I'll make this list bigger. If you have any hints or tips of ongoing discussion on Drupal monitoring, please post it as a comment. Thanks.