WP Sentry allows WordPress authors to grant access to individual private posts to users and groups of users.
(20130223) Yes! The original author of this plugin: Pete Holiday has decided that he's no longer able to support this, but Steve Weigold (sweigold) has offered to take over support. We're a little slow getting things up to date, but we are working on it! The new plugin homepage can be found at: WP Sentry Homepage.
Because all of the posts are still listed as Private, WP Sentry is fail safe. If you disable WP Sentry or it stops working for any other reasons, only site administrators will be unable to access your private posts.
Take those on over to the WP Sentry Homepage. I'd love to hear either or both.