Sounds familiar..Our cat does the same thing, but only to my 7 year old daughter. He adores her to "death." Wherever she is, he is there too. He only lets her carry him around and he only gives her, what we call, head-butts (when he bumps her on her forehead with his head).
He's gotten to where, if we don't lock him up in his room at night, he'll bite her feet or fingers during the night. This saturday, he wanted so desperately to wake her up that he bit her earlobe.
I don't know, but I'm thinking they either want complete attention or its a sign of affection.