We will upgrade our official sandbox to Confluence 3.2 soon, but if you want to try out the new features now already, just leave a comment below (this server runs the Beta 2 release already). Use the RTE and type "!" or "[" to activate this cool feature. Or check the Confluence 3.2 Beta Release Notes for more in-depth information on how to use Autocomplete and the new Links dialog.
Oh, and the nice styling you see here is actually part of Confluence 3.2 too.
Important: You must be logged in to use autocomplete
We deliberately disabled it for anonymous access, since anonymous users have no way of storing settings (e.g. to disable the feature it they didn't like it).