[biot]

  > In a default setting, Siri let's [sic] a complete stranger see
  > your calendar on your passcode locked iPhone 4S, as well as get
  > contact information, make a call and send texts and e-mails.

A complete stranger could also steal your phone. Solution: don't leave your phone accessible to complete strangers.

[baddox]

I think the point of the passcode lock is so that if someone does steal your phone, they won't be able to get any of your personal information from it (provided they're not tech savvy enough to do a relatively easy brute force).