i'm guessing this is more for Exchange 2000 users, since it has the built-in messenger functionality that was removed with Exchange 2003. that is, unless you are able to enable Windows Messenger for corporate-only, which i'm sure can be done, although we don't have an enterprise IM client here sans Office Communicator (which hasn't been deployed widespread here yet).
and speaking of, they are also enabling Office Communicator for BlackBerry at some point later this year, from what i've read/heard. a beta is still not out for it, though, but when it is, i will post up some screenshots of it in action.