One of our platties is sick, so as I was reading through more posts on Usenet, I found something that peaked my interest. "How to kill a goldfish humanely"
In short, putting a fish into the freezer is not a humane way to kill a fish, as fish are cold blooded animals. The colder it is, the longer you are prolonging its life for -- how does a slow cold death sound to you? not very fun.
Methods that might be a bit better are:
Plastic bag dashed against rock.
Exposure to dry air.
Moral of the story, don't stick dying fish into the freezer; you will upset the fish gods, and you don't want to do that now do you?