Trivia: Henry Cavill was considered to play Superman. He'd later go on to play the part in Man of Steel and subsequent movies.
Trivia: While the film received generally positive reviews and decent fan reception upon release, it gained a moderate amount of backlash in the years since its release due to its slower pace and lack of action in the first half. Director Bryan Singer has admitted that the film is a bit too heavy on drama and light on action, and has stated that if he could go back and re-do the film he would have cut out most of the first act to quicken the film, and would have begun the movie with the famous "plane rescue" sequence to grab ahold of the audience's attention early on.
Answer: There are two stories: a) the suit was made by Martha Kent from the blankets which came with Superman when he was a child and these blankets got superstrenght from the sun too. b) Superman generates an aura which protects him and his suit against such forces. Both stories can be true: either the suit itself is weakend by the kryptonite or Superman's aura was weakend by it. Currently it is said that the blanket-story is the correct one. Also, they don't actually cut it off him - it appears to open along a predefined line (coupled with someone exclaiming "got it!"), presumably the method Superman uses to get into the suit himself.
Bjoern_Buller