Do you mean you want to have a security feature to enter before your iPod will start (like a cell phone for example)?
Since you can't rack up a 3-digit phone bill calling China on your iPod, there is no such feature right now
But, being a paranoid security freak myself, I can see the benefit of having a security feature. I have a sneaking suspicion that with iPod Software 2.0, which will include a Notes feature, they might incorporate some kind of security system, since notes could be kind of sensitive.
Too bad iPod's OS isn't like PalmOS, and a little more 'hackable'.