#define radians(degrees) (degrees * M_PI/180)

UIImage *rotate(UIImage *image) {
  CGSize size = image.size;;

  UIGraphicsBeginImageContext(size);
  CGContextRef context = UIGraphicsGetCurrentContext();

  // If this is commented out, image is returned as it is.
  CGContextRotateCTM (context, radians(90));

  [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
  UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();

  return newImage;
}

Alguma outra coisa pode estar errada? Fora de idéias.