CirclingTheSun: I'm also a linux guy and would choose Linux over MS any day, but I'm afraid to say Linux has got a long way to go for end users. They are barely managing to use windows as it is. Although everyone knocks MS - they do have some good packages - take Exchange for example, an excellent piece of software for the corporate, and they've done a good job of getting a usable OS for the end user.
sky.akash: You pretty much have to block everything - you can't have users running wild on the net. With IM there's also no logging of who goes where etc etc. If something happens on IM and you need to take it to court there's no information trail. This together with information leakage this can only lead to IM being blocked.