As I didn't find any munin plugin to monitor my miners' hashrate, i just contributed two plugins which support slush's pool and btcguild.
You can find them here
https://github.com/deveth0/contrib/tree/master/plugins/bitcoin and hopefully in the main repository as soon as the pull request is accepted.
Usage is quite simple:
ln -s /path/to/plugin/slush_hashrate_ /path/to/munin/plugins/slush_hashrate_YOURAPIKEY
Hope this helps somebody
