Would you like to limit who can read your blog?

The audience in tonight's TechNet event asked whether there's an easy way to restrict who can access your blog - particularly if you'd like to limit the audience to be friends of your Children.

I know that some blog engines provide this granularity (such as www.blogger.com) - which ones do / which ones don't?

What's your experience of trying to restrict who can access such a blog?