Scott Pfitzinger at Biblotechweb has a good write-up of using instant messaging in reference. He writes:
Instant Messaging is an incredibly easy and ubiquitous form of communication. There is definitely a place for it in practically every library. Whether itâ€™s to provide patrons with an alternative method of communication or to facilitate internal communication, IM can provide a great service to your library. It will not replace other methods of communication, such as phone, email, or live conversation, but by providing another communication option to patrons who are increasingly coming to expect it, you help maintain the viability of the library as a quality information resource.
Amen brother Scott, amen.