I have already enabled this. It is really cool when there is a really long thread in your email and you get about half way down and then the phone rings… happens to me all of the time!  With this, I can mark that I have read to this point so I can open it later and start where I left off!  LOVE IT!

There’s a new feature in Gmail Labs that will help with this. When you enable Mark Unread From Here from the Labs tab under Settings, you'll see a new “Mark unread from here” option in the drop down menu found in the upper right-hand corner of messages.

via Official Gmail Blog: New in Labs: Mark unread from here.