Anything that's writable, including firmware, can be persistently hacked, with the hack surviving reboot.
I remember some Sun gizmos meant for schools that booted only from CD. (not the usual culprits, spare you that)
And many linux distros will do so, CD/DVD or memory stick (write-protect it.) I keep asking Klaus Knopper to update Knoppix. Oh well. And of course, there's TAILS.