Think back to the last week or month of IM conversations you've had. Perhaps you have archiving on and can actually read them. Now, I want you to take a megaphone and broadcast those conversations while at the same time emailing them to everyone in your address book. Don't forget to print a copy for your coworkers and boss. Quick poll of hands, who's willing to do this?
Except for you exhibitionists out there, there's probably not too many hands in the air (you can put them down now). What not many people realize (and for you techies, tell your non-tech friends) is that using an IM tool is equivalent to broadcasting your conversations. Now, not every company is listening to those conversations, but all it takes is a widely available network sniffer and an inclination.
IM clients send messages in clear text over the network. So if big brother feels like listening in, no problem. Some companies employ tools for archiving IM conversations by their employees and analyzing them to see if corporate secrets are being leaked.
If you're going to talk about sensitive topics, I'd recommend using an IM client that encrypts communications. The Jabber protocol provides encrypted communications. The client I'm using is Pandion (email@example.com).
You're employer does NOT need to know about that suspicious rash you acquired after your business trip to Asia.