Some, yes! It really does depend on whether the IS Dept. decides to block it or not... most chat sites use a protocol called ICQ to exchange messages, and that's a type of traffic that firewalls can be set to selectively block.
As an example, at the place I most recently contracted, the IT folks were having a big discussion on whether to let AIM through or not... it and other ICQ traffic was blocked, and they decided to keep it that way.
As another example, Dell, where Rose works, doesn't block AIM. That's nice because when I'm home we use AIM to keep in touch (she can't make personal calls from her desk.)