I am a bit behind on FamilySearch, I have been watching the new images, but missed the who "Source Box" thing until the other day. I was doing some research on my German Heritage and putting my info in the database, when I noticed an option on the page called Source Box.
First you had to login to FamilySearch but then you just click "ADD". Considering at the time I was matching about 20 or so names to my family name it was a nice bonus to add the source so I can always go back to it and double check my information.
After finding out about it on my own I decided to see when it was added, which was May 25, 2012 so not to far behind. I did some more reading about Source Box and it looks like it has the ability to add the sources to you family tree on FamilySearch.
I do not have a tree on FamilySearch so I cannot verify how it works, and if it is a worthwhile thing to do while working on your tree at FamilySearch.
You can also create folders in your Source Box, so you can have it organized by country, or surname. It looks like its a bookmark back to the information you found. I think that I am going to be using this a lot more, wish I found it earlier.
Now I have to go back to all the other pages that I found on FamilySearch and add them to my SourceBox for future references.