In the “good ol’ days” of WordPress—which, in comparison, were good-but-not-great—you could create a new layout in a separate file—I always used “testindex.php”—and, if it knew how to connect to the database, you could check out your style changes on a semi-private basis before taking them public. Since the move to 1.5, though, there’s been no clear-cut way to make changes to your theme without editing them in public. [For example, I've been tweaking my theme live for all to see. ]

Now, Ryan Boren has developed a Preview Theme Plugin that allows all admin-level users to preview a theme before it’s pushed live to all users. Tres awesome. Kudos, Ryan!