IIRC the story is that Mark Hamill had a car accident in between IV and the filming of V starting and the wampa attack was a device to facilitate this.
Jeez, there's a thread all about this on Snopes. I'll let the big boys hash this one out.
http://msgboard.snopes.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=75;t=001029;p=0