If I remember the presentation I saw on this, some don't even verify firmware updates.

http://ids.cs.columbia.edu/sites/default/files/CuiPrintMeIfY...

http://engineering.columbia.edu/can-you-trust-your-printer

Not if the owners don't password protect anything. Without a valid login it shouldn't print anything, unless that login were exploited, but I don't see any mention of a password being bypassed.