I actually just tried this and it in fact is a clear bug with Siri. Now anyone who gets a hold of my phone can post bad things on my behalf (ok not entirely true…read on).
The one difference here is if you have a passcode lock on your phone and the timeout has not been reached you can do this. If the passcode time out has been reached then you will not be able to do this. So really the problem is for people that have pranksters around them, if your phone is not completely locked then they can easily post things on your behalf.
After playing around I realized my IBM security policy prevent Siri from being used on a fully locked phone. Unfortunately for iOS 6 users with no corporate security policy installed can’t even disable Siri from working while the phone is locked. This is really bad, and my wife was shocked when I showed her I could write on her Facebook wall without getting into her phone.
Check out my updated post here.