"""Change a ChoiceField select widget into a hidden field if the right GET variable is passed"""

""" code for your view """
designation  = request.GET.get('d')
choice       = {}

if not designation:
	form = Form()
else:
	"""Setup the designation field as a hidden field if the right get variable was passed"""
	
	form = Form(initial={ 'designation': designation })
	from django.forms import widgets

	for item in form.fields['designation'].choices:
		if str(item[0]) == designation:
			form.fields['designation'].widget = widgets.HiddenInput()
			choice['designation'] = str(item[1])

return render_to_response('my-form.html', { 'form': form, 'choice': choice }, context_instance=RequestContext(request))


""" code for your template """
<p>{{ form.designation.label_tag }}{% if choice.designation %}{{ choice.designation }}{% endif %}{{ form.designation }}</p>