Siri’s a lot of things but one thing she can do for sure is allow access to some of your information who doesn’t have your passcode.
For example, on a locked iPhone, if you ask Siri for a number of a contact, she gives them to you. She gives out contact information, calendar appointments, reminders, weather, and perhaps a few more. She used to allow access to everything in previous firmwares – you simply had to ask her to search for something and she would launch Safari, which anyone, regardless of whether the phone was locked or not, could access the phone.
With iOS 5.0.1, Siri asks for a Passcode if you ask her to either check mail or search the web, which is good. However it can still be unsettling for those that want to conceal their identities completely from prying eyes. While I could see benefits of Siri being able to tell the person who the owner is so they could contact the owner, but the truth is, I think more iPhones are forever-lost than returned to the owners.
Fortunately, there’s a way to turn Siri completely off when it’s locked with a passcode.
Settings -> General -> Passcode Lock -> Siri “OFF”
This will prevent Siri from being activated from the lock screen if your phone is passcode locked. Not being able to access Siri while the phone is locked poses some degree of inconvenience but I think it’s worth it if you’re concerned with security.