Hey Alumnus,
Singing what you play is invaluable. It strengthens the bond between what you hear and your fretboard awareness. The key is to sing AS you play. Basically, improvise and match the pitches with your voice a la George Benson or Otiel Burbrige (bassist extrordinaire). Since guitarists tend to think in shapes and fingerings, this is a great way to break that habit and co-relate your inner musicality to your physical execution.
Also study intervals (the distance between any 2 notes) as this will allow you to develop relative pitch. Relative pitch will allow you to name notes by ear simply by their "musical flavour"...providing you have knowlege of the underlying harmony and what key you are in! This is not PERFECT PITCH (the ability to name any note in solitary or succession/ backing harmony or not). This post would be way to long to go into how to study intervals so, do some internet research and get a theory teacher for best results.
Hope this helped!