There are issues for companies, some on the security side and some on appropriate worker behaviour. The issue of whether or not there is a valid reason for a worker to have instant messaging at work is a double-sided sword. For example, IM has been used around here to save on long-distance phone bills; however, at the same time, IM has also been used around here to chat with friends and girlfriends to the point of alot of work time being lost. For this reason, alot of companies have built out a 'use of company resources policy,' alot of which restrict usage of instant messengers.

It has been my experience, that alot of companies that have restricted access to IM's have done so not only from the perspective of the potential for lost work time, but also for the numerous vulnerabilities found in almost (if not all) IM's over the last couple of years. Things like worms and trojans, propogating through IM and point-to-point connections, are a concern for many security minded organizations. With that being said though, I have yet to see anything that was entirely foolproof (and not detrimental to normal usage) in blocking IM because most, if not all, IM's have a way to configure their port usage to allow communications out when the default methods have been blocked (or even better, using a java client over the web).

Neb