Is any of this behavior new? I briefly installed an older version (2.62) and the behavior of the wheel within the Tag Panel appears to be the same as in 2.66.
Edit: Wait... I do see some issues. I think it has to do with attempting to fix issues reported previously. It appears that once you begin editing one of the fields in the tag panel, which are all combination text input/drop-down menus, it's difficult to get back to where you can use the mouse wheel in that field. I've done it, but I can't say exactly how. IMO it should go back to wheel mode as soon as the user clicks the down-arrow to open up the drop-down menu.
If we're (also?) talking about the scroll bar within the Tag Panel itself, that behavior doesn't appear to be new. It looks like there has never been a way to take focus away from one of the fields in the Tag Panel, so no way to use the mouse wheel to scroll the Tag Panel itself.