Cisco IP Phone firmware images now contain a Cisco digital signature to provide authentication for improved security. Once firmware version 5.1 is loaded onto a phone, all subsequent firmware versions will be checked for authenticity.
Firmware images prior to release 5.0 do not contain a Cisco digital signature and therefore once version 5.1 is loaded onto a phone, all prior firmware releases will be rejected. Only version 5.0 will be accpected as it contains a Cisco digital signature.
For additional release 5.1 upgrade information see the release notes.