Default filter in Django admin

In order to achieve this and have a usable 'All' link in your sidebar (ie one that shows all rather than showing pending), you'd need to create a custom list filter, inheriting from django.contrib.admin.filters.SimpleListFilter and filtering on 'pending' by default. Something along these lines should work:

from datetime import date

from django.utils.translation import ugettext_lazy as _
from django.contrib.admin import SimpleListFilter

class StatusFilter(SimpleListFilter):
    title = _('Status')

    parameter_name = 'status'

    def lookups(self, request, model_admin):
        return (
            (None, _('Pending')),
            ('activate', _('Activate')),
            ('rejected', _('Rejected')),
            ('all', _('All')),
        )

    def choices(self, cl):
        for lookup, title in self.lookup_choices:
            yield {
                'selected': self.value() == lookup,
                'query_string': cl.get_query_string({
                    self.parameter_name: lookup,
                }, []),
                'display': title,
            }

    def queryset(self, request, queryset):
        if self.value() in ('activate', 'rejected'):
            return queryset.filter(status=self.value())    
        elif self.value() == None:
            return queryset.filter(status='pending')


class Admin(admin.ModelAdmin): 
    list_filter = [StatusFilter] 

EDIT: Requires Django 1.4 (thanks Simon)

I know this question is quite old now, but it's still valid. I believe this is the most correct way of doing this. It's essentially the same as Greg's method, but formulated as an extendible class for easy re-use.

from django.contrib.admin import SimpleListFilter
from django.utils.encoding import force_text
from django.utils.translation import ugettext as _

class DefaultListFilter(SimpleListFilter):
    all_value = '_all'

    def default_value(self):
        raise NotImplementedError()

    def queryset(self, request, queryset):
        if self.parameter_name in request.GET and request.GET[self.parameter_name] == self.all_value:
            return queryset

        if self.parameter_name in request.GET:
            return queryset.filter(**{self.parameter_name:request.GET[self.parameter_name]})

        return queryset.filter(**{self.parameter_name:self.default_value()})

    def choices(self, cl):
        yield {
            'selected': self.value() == self.all_value,
            'query_string': cl.get_query_string({self.parameter_name: self.all_value}, []),
            'display': _('All'),
        }
        for lookup, title in self.lookup_choices:
            yield {
                'selected': self.value() == force_text(lookup) or (self.value() == None and force_text(self.default_value()) == force_text(lookup)),
                'query_string': cl.get_query_string({
                    self.parameter_name: lookup,
                }, []),
                'display': title,
            }

class StatusFilter(DefaultListFilter):
    title = _('Status ')
    parameter_name = 'status__exact'

    def lookups(self, request, model_admin):
        return ((0,'activate'), (1,'pending'), (2,'rejected'))

    def default_value(self):
        return 1

class MyModelAdmin(admin.ModelAdmin):
    list_filter = (StatusFilter,)

Took ha22109's answer above and modified to allow the selection of "All" by comparing HTTP_REFERER and PATH_INFO.

class MyModelAdmin(admin.ModelAdmin):

    def changelist_view(self, request, extra_context=None):

        test = request.META['HTTP_REFERER'].split(request.META['PATH_INFO'])

        if test[-1] and not test[-1].startswith('?'):
            if not request.GET.has_key('decommissioned__exact'):

                q = request.GET.copy()
                q['decommissioned__exact'] = 'N'
                request.GET = q
                request.META['QUERY_STRING'] = request.GET.urlencode()
        return super(MyModelAdmin,self).changelist_view(request, extra_context=extra_context)

class MyModelAdmin(admin.ModelAdmin):   

    def changelist_view(self, request, extra_context=None):

        if not request.GET.has_key('decommissioned__exact'):

            q = request.GET.copy()
            q['decommissioned__exact'] = 'N'
            request.GET = q
            request.META['QUERY_STRING'] = request.GET.urlencode()
        return super(MyModelAdmin,self).changelist_view(request, extra_context=extra_context)