Audio issue (headphone output no working)

Anyone else had this problem... after installing Lion today I cannot get audio out of the headphone socket, when I insert headphones (I've tried two pairs, both of which I know are working fine) the audio just continues to play via the built in speakers. Also, in the system prefs the headphone socket doesn't appear in the list as an output option, only Internal Speakers? It's as though the system doesn't recognise the headphone socket

any ideas greatly received

System

MacBook Pro 2.8 Ghz Intel Core 2 Duo, running 10.7 (Lion)