If you highlight the text that you are interested in then type a note, you get both the highlighted text and the note in MyClippings.txt.
So, for example, in this PDF on Page 20 I highlighted "Adding a role" and then typed the note "note this".
MyClippings.txt has this in it:
Windows_Server_2008_R2_e-book
- Highlight on Page 20 | Added on Monday, January 30, 2012, 07:24 PM
Adding a role
==========
Windows_Server_2008_R2_e-book
- Note on Page 20 | Added on Monday, January 30, 2012, 07:24 PM
note this
==========
I think that to be any more spatially referenced than that you would need something that implements a full version of Acrobat with its note taking functions - a tablet for instance.