Mongrel_rails and monit are not the best of friends. It's difficult to get them to work together.
For example, this is the error I got in my monit logs when switching to a new mongrel_rails command that cleans up stale pids:
'mongrel_1' process is not running 'mongrel_2' trying to restart 'mongrel_3' start: /usr/local/bin/mongrel_rails 'mongrel_4' failed to start
To fix it I added the following start_command to the monit configuration:
/usr/bin/env PATH=/usr/local/bin/:$PATH mongrel_rails cluster::start -C /var/www.... --clean --only 8000
The problem is that monit overrides the PATH environment variable, so it won't find mongrel_rails unless you tell it where to find it. Monit also contains a bug which doesn't tell you why it can't start mongrel_rails, but that's another story...
Note that I'm using the --clean switch which will startup the mongrels even if a stale pid exists.
In fact I got so tired of the whole mess I wrote a plugin that generates a working monit configuration for mongrel_rails from one or more mongrel_cluster.yml configuration files.