Are there any legal issues with iTunes sharing? I responded to this question on Super User, and if you look at the comments, the point was brought up that iTunes sharing may or may not be legal.
The iTunes Store Term and Conditions are available in almost any country and can be found here. The Terms and Conditions, as outlined by Apple, should address your broad question.
As Daniel has raised the issue of "sharing in the workplace", it is a point of contention that one can argue what "sharing for personal, noncommercial use" entails. Does that cover other parties that listen to the music? Do you have to be in the room (or within earshot) with them to have the sharing fall under "personal" use?
I'm inclined to think that sharing your library at work while you are in the office would not be something that would see you land in court. I can't see a judge in their right mind agreeing to anything more than a C&D, if that, under such circumstances.
You may consider reaching out to the Electronic Foundation Frontier for a more fleshed out answer.
At least in the US agreement, Apple only authorizes people to use iTunes content for "personal, noncommercial use." Nothing I find in the agreement licenses the user to share their content with others at the workplace. So yes, there are potential legal issues.