The proper way to do this is of course to use the ModelAdmin classes that were introduced with the merge of the newforms-admin branch.

Declare your model as usual in models.py without the inner Admin class:

class Article(models.Model):
    title = models.CharField()
    slug = models.SlugField()

Then declare the ModelAdmin class in admins.py. This is where you set properties regarding the admin interface, it replaces the old-style inner Admin class:

class ArticleAdmin(admin.ModelAdmin):
    prepopulated_fields = {"slug": ("title",)}
admin.site.register(Article, ArticleAdmin)

This is the correct way to do it. Rip out the inner Admin-classes and create new ModelAdmin-classes. Read the new and updated documentation for more on newforms-admin.