Since you are on their dime while there and on their equipment, they have every right enforce the non-personal use policy. That being said, I agree with Lisa, what risk are you willing to take? If you decide to continue using it for personal use after receiving the email, then it is for you to be accountable for any disciplinary action taken against you, including termination. Now you are a volounteer to any dicipline, not a victim of misunderstanding or corporate red tape. Even if your immediate supervisors say it's ok, but the policy comes from higher up, the ultimate responsibility comes to you. (I know this from 15 years at a big corporation)
While I may not always agree with the policy, there is a reason for it.
If it were me, I would minimize the risk and not do it. If they were paying you to surf the web, they would have put it in your job description
