A neat article courtesy of bruce_schneier
about how the XBox was hacked:http://www.xbox-linux.org/wiki/17_Mistakes_Microsoft_Made_in_the_Xbox_Security_System
There are many, MANY interesting insights here, but my favorite one is how they grounded out the A20 line in order to make the CPU boot from the address FFE
F:FFF0, instead of the usual protected/encrypted/hardened address FFFF:FFF0. This allowed them to utterly and completely pwnZ the XBox hardware with almost no effort at all.