In form, that displays the checkbox do something like this:
def __init__(self, *args, **kwargs):
super(FormName, self).(*args, **kwargs)
if self.instance and self.instance.checkboxfieldname != None: #we check if this form has instance and if checkbox :
attrs = {'checked':'checked'}
else:
attrs = {}
self.fields['checkboxfieldname'] = forms.BooleanField(widget = forms.CheckboxInput(attrs = attrs))
should work.
alan