Corrected entry: In the London scene where Reed ties the ferris wheel in place he accidentally touches Johnny and inherits his power. However, when they switch back again Reed still has his body suit intact even though it was enough to burn Sue naked earlier in the movie. The capability of their suits are only immune to their specific powers.
Tanngrisnir
19th Aug 2007
Correction: It is implied in the first movie that all of their suits are capable of withstanding each of their abilities. That means that in the London scene Reed's as well as Johnny's suit remain intact despite inheriting the other's power. As far as the other scene is concerned Sue wasn't wearing her suit when she accidentally swapped powers with Johnny so her normal clothes obviously burned away.
Tanngrisnir