The mDNS fix tackles the Dan Kaminsky DNS vulnerability that sparked controversy over the pace of Apple's patch releases... yet more proof that the iPod touch is a teensy little computer, with all the risks and challenges thereto. You can review the security notes for the update at Apple's security site, and of course you can download the update through iTunes.
Also updated for security purposes today was the Bonjour for Windows package, now at version 1.0.5. This utility, which gives XP and Vista machines access to zero-configuration network resources such as printers or Mac OS X web sharing, now includes a couple of DNS-related patches including one for the vulnerability noted above. See here for the full details; Bonjour for Windows is downloadable from Apple as well.