If at any time you want to restore to stock firmware, download it from
HERE.
You must find your corresponding version and make sure it's CDMA.
just start iTunes and hold shift + press "update" it will ask you for the IPSW file you just downloaded.
You may need to do this on a failed jailbreak, or when 4.3 comes out and you don't want to update, but want to flash stock, you would use this. You would also do this if ever your phone is "bricked"