IM's communication is too quick, exacerbating etiquette issues that plague email.
Since each IM protocol uses a different port, a different server, and a different text format, filtering and monitoring IM traffic is very difficult. The potential for abuses is large. Inappropriate chatting, divulging of the company's inside information, time wasting, sharing viruses, identity spoofing. The list goes on.
I often make the point “who runs the servers and do you really want them know what your employees are talking about?” There are more than a few companies that would consider Yahoo, Google, AOL or Microsoft to be competitors.
There's an answer to all of these problems; Enterprise IM. Enterprise IM is simply an IM server that exists behind the firewall and is officially sanctioned by the company. And, there are a lot of options available for Enterprise IM servers. IBM makes SameTime. Microsoft sells an MSN-compatible server with its Exchange product suite. I think AOL has an offering. My favorite, by far, is Jabber.com's Jabber server.
AIM and Google Talk integration, so your employees don't need to download a non-supported program to talk with their clients.
LDAP and Active Directory integration, so it's a good enterprise client.
A single, unified access point for all IM communications, making monitoring much easier.
Widely available, high quality, free SDKs for integrating IM and presence features into applications.
All internal company communications remain internal.
An open protocol with an open implementation.
The ability to federate the Jabber server with other Jabber servers both inside and outside of your network with no loss of control or privacy.
IBM's SameTime is a close second. It has many of the same features, including an open protocol since it uses SIP, and a high quality SDK. But, SameTime does not include an AIM gateway, so your employees will download the AIM client themselves, which leads you right back to the monitoring questions.
My product, Reliable Response Notification, supports all of these clients. We want to spread our net as widely as possible. But, when someone asks me which IM I suggest, I never hesitate to suggest Jabber.