Google Reader got a new “conversation feature” that allows you to have a private discussions on shared items with your friends. Comments can only be seen by friends of the person who originally shared the item. And don't forget, you can always read and add comments on your iPhone.
To add a comment, click the “Add Comment” link at the end of your friends’ shared items. The new “Comment View” menu item in your “Friends’ shared items” section displays all items with comments. (Note: Commenting isn’t currently available in the “All Items” views.)
More info: Google Reader Blog