IIRC, Castiel is cut off from heaven because of his disagreements with Zecheriah. At least I think so.
I personally would've chosen Tabris to be Castiel's name. Angel of free will, and with all the ho yay between him and Dean, it doubles as an Evangelion reference. (Is there anyone who does not have some subtext with Dean, though?)
I read that Castiel is the angel associated with Thursday, so it made sense, since Thursday was traditional SPN day until this year.