I get the arrows moving up & down, then an X because 4G is not very good in Atlanta area (not even at the Sprint store) and nonexistent at my house. Even with an Airave, no 4G but that is by design by Sprint. If I think about it, I resent paying for 4G that I can't use but that is the way it is.
Before the ICS there was a different icon but 4G has been so sporadic I had not noticed before you mentioned it.