As it turns out, printers, in particular (at this point) HP Laserjets, are entirely open to a devastating hack. And it doesn't end there: embedded devices, even in otherwise secure environments (think anything on a network like a Blu-Ray player, a coffee machine, and just about anything else which downloads firmware updates) is equally vulnerable, since almost none of it requires a digital signature.
We especially like the part where you can use a laser jet to start a fire remotely...