I've just checked in a new feature which lets you limit the access (view/edit) to a page in a simple way.
Please play with the new access tab and report me if you find some bugs. You shouldn't because I tested the feature already quite a lot but because it is a security-relevant feature it should be well-tested.
If you choose that a page should be visible only to logged in users or to a certain user group, the link disappears also in the navigation.
Administrators can always view/edit the page, even if the access is restricted to some other group.
Please make sure to call ../db/build?flush=1 after checking out the changes.
P.S.: If you have installed the forum module, you will notice that you now have a lot of options in the access tab that are not easy understandable. I'm aware of that problem and I think the forum module should be changed to use the new SiteTree features.