I was experimenting with censornet over the weekend, and changed the GPO proxy settings to point to the censornet box. Took a little fiddling to get it going, finally blanking the local profile proxy settings did the trick.
Changed it back, and no machines were picking up the correct proxy and getting out to the web! Setting a local proxy again has fixed it for the kids, but staff are still unable to get out.
Staff are on a child domain and no settings were changed there!