orange box ntsc works on pal!?

a few days ago i recieved the orange box through the post from my rentals service and i noticed the disc said ntsc and had T-M as the rating. as i live in the Uk i thought it was strange but i tried it and it worked! i thought you could not play usa games in the uk so i was shocked this worked.  anway my questions are:1. has this happened to anyone else2. is it just this game or can i start importing games from the usa for low prices  any help appreciated