There is a nagios plugin available on the MMM's google-code page, but if you didn't find it yet, here it is:
You can call this plugin over nrpe. I'm already working on to fork a version which more useful with passive checks.
This plugin was developed by Ryan Lowe (Percona).
There is no centralized authentication for mysql. You have to set up your user accounts individually for each of your mysql servers. You can use replication, but that would mean, your master servers must have the same authentication privileges as your slaves have. What about security?
So, here is a little workaround that I did and I can't find the same anywhere.