Dd started doing this for a while, and it became a game. She would mimic anyone who shook their head 'no'...even when the cats shook their heads. Not that they were saying 'no', but they do shake their heads sometimes...
Anyway, I think she just thought it was funny. It makes her smile and laugh when other people do it back to her, but I was also concerned when she seemed to be doing it a LOT. I was thinking she might have tourettes syndrome.
She's almost 11 months now and she doesn't do it as much. Usually she'll do it as part of a conversation. "Baby, are you ready for bath time??" *baby shakes head*
I'm sure she doesn't realize it means 'no' yet, but it makes us laugh.