To use it, launch Disk Utility, and you will find the Erase tab has a new Options button. Logically, you must have a non-system disk connected to use it, as it was greyed out on my Powerbook until I connected an external firewire drive and selected it. When you click on it, a pop-up box warns you that this will significantly increase the disk initialization time, and there is just a single check box "Zero all data" with a small explanation "Writes zeros to all sectors of disk. The disk cannot be recovered once it starts."
I didn't actually try it yet; note this doesn't meet more stringent DoD-mil spec standards which require things like multiple passes and writing different patterns, but it sure is better than nothing for the (cheap) security conscious (paranoid?).
Yes, there are numerous other more sophisticated options, e.g. BITcom's Xclean, BCWipe for UNIX (anyone compiled on OS X yet?), PGP8 (?) etc., but this one is simple and built-in.

