Hi All, (I am not a lawyer but...)
I am taking a business law class right now

,
I know that at least with US law the following applies: (I think the laws would be pretty similar elsewhere too)
-There is an established precedent for reading email assuming that they are using your email server, you can read whatever you want.
-BUT, with any other electronic surveilance (phone, IM, etc.),
there must be something in the employee handbook saying that it is subject to surveilance.
If employees sign something acknowleding that, you really shouldn't have any problems. (Of course that depends on local contract and labor laws too) Then it just becomes more of an ethical question.
-Check your company's privacy policy and see if any private communication at work is really considered private.
-Some companies have gotten in trouble for saying that electronic communication is private but then later used it as grounds to fire someone. (Michael A. Symth Vs. The Pillsbury Co. 914 F. Supp. 97, 1996 U.S. Dist. LEXIS 776) In this case, someone was making some death threats
After that long winded thing...
I would recomend etherreal.