my ds totally did this too...i don't recall how old he was when it started, but 6 months sounds about right...i remember asking dh if he thought that was normal because i was a little concerned, too! once i stopped fretting over it, i thought it was pretty cute (like every other new thing ds does! )
Yep, my LO did it, too. Now that she has a little better control, she's stopped hitting herself, though she still waves sometimes. Now her favorite thing is to kick her leg while nursing a la Rockettes!