A few people have asked me for the code. Ok, here it goes.
First, get qmrtg.
Then you need this: qmunin.tar.bz2
Then build and install it. You may need to modify the sources, depending on just how your qmail works. There are some example munin plugins included (in spanish, I am not translating them ;-), which 99% surely will not work for you, but they are simple shell scripts, so you should be able to hack them.
Creating a real release of this is just useless, because everyone's qmail logs look different, so take it and hack it.
And that's it.