I am a big fan of /. The nerds there sometime can get on you, but sometimes you stumble upon a real gem, such as this - (I quote mi from this discussion)
In short, you have to remember, that you (an administrator) exist for the benefit and convenience of these people, not the other way around. So if they want to be able to access their webmail, you must have a much better reason than "you may get a virus" to deny it to them.Nice!
I bet, more productivity is lost, when an employee brings in flu and half the office gets sick. But no one is advocating forcing people to take vitamin C and wear scarves, right?..