observe_field, :function, same value

Hey all I'm having problems getting the correct value of a checkbox. I
have an observe field that calls a javascript function whenever the
checkbox is changed. But for whatever reason the checkbox value always
seems to be 1. Perhaps I'm using the wrong syntax to get the value?...
My code is below, any help would be greatly appreciated.

The value of a checkbox doesn't change. You need to be checking
whether its checked property is true (or use something like prototypes
$F() method which abstracts away that sort of concern.

Fred