Starting today, users who have opted into the Steam Client Beta will see a new version of the shopping cart when checking out on the Steam Store. These users will also be able to mark games in their library as private. Shopping Cart Updates The Steam Store's updated shopping cart includes some handy new features: Inline gifting - just in time for the holidays, you can now purchase gifts for multiple friends (and yourself) without having to make multiple purchases.
I just ran a quick check and private games (whether installed or not) are still shared. Seems like a weird oversight.