Update: I wrote a theme switch plugin that changes the template for logged in users. It's available here.
I like to tweak my wordpress theme. And I'm far too lazy to set up a second blog just for development. So I'm looking for a solution to work on my template without distracting visitors. I think one day I'll write an extension that is a little more powerful than this trick, but it's a start.
The easiest case is that you sometimes want to play around with your CSS styles. In this case, just add something like the following example to your template header. Put it where you include your CSS, and only logged in users will see your experimental CSS.
Of course, you can be more specific than showing any logged in user the testing style. Just compare the $user_ID with some specific ID.