As you will have noticed by now if you've entered an instance in progress since patch 3.1, Blizzard's new binding notification system is in effect. Whenever you join an instance in which bosses have already been killed, you get a warning that staying in there will bind you to that instance -- thus hopefully preventing you from accidentally sticking to an instance you don't like.
However, as a few players have noted, there's still some functionality missing there. While the notification will tell you that bosses have been killed, it won't yet tell you which ones, so a malicious or confused guild could invite you a Naxx in which only one wing has been downed, when really all they've got left is Sapphiron and KT (or no bosses at all, if they really wanted to ruin your week). Fortunately, Zarhym agrees, and says that discussion about adding a little bit to the notification is incoming. It seems a little bit weird to just have a list of killed bosses on your screen when you first walk into the instance, but maybe they could add some sort of "boss tracker" to the instance maps, where you could see at a glance what was going on in any instance you join.
As a few other people say in the thread, most of you may never have seen this notification pop up -- if you only join instances where nothing is killed yet, you'll never see it. But for those of us who take boss kills where we can get them (in PuGs and guild runs where an opening appears), it's a nice feature to have.