For awhile I used this eMember plugin (until I found WPMU Dev of course!). One features I loved with it was the ability to favorite posts when logged in and to set up a page where your favorites would be. You can also un-favorite them from there.
This would be a sweet plugin on it's own. Just another suggestion!
Here's the demo - http://www.tipsandtricks-hq.com/eMember-demo/