Cron job to monitor RBL listings

Since I'm now running my own mail server, I want to know if my server's IP ends up on any prominent RBLs so I can, you know, [post an angrygram](http://pigeonsnest.co.uk/stuff/crapstuff/sorbs.html).

There are some free services online to do this:
* http://mxtoolbox.com
* http://www.rblmon.com

But since this is a relatively simple check, perhaps we should include a cron job that checks once a day. Here's a nice-looking script:
* https://github.com/DjinnS/check-rbl