Discussions were added to Google Docs and Presentations earlier this year. Now you can add discussions (comments) to spreadsheet cells. To create a discussion, click on a cell to select it and from the "Insert" menu choose "Comment"
If a cell has a discussion in it, you'll see an orange triangle in the upper right corner of the cell and when you click on the cell, the discussion will pop up.
For more info and details read on at: http://googledocs.blogspot.com/2012/09/google-spreadsheets-now-with-discussions.html