I think this is the way to go. I am taking this approach for a multi-player game I'm developing - you can play without registering, but you can register afterwards to save your score.

My primary project, mystorespace.com, takes a similar approach. We have a fully functional demo accessible without an account (you will be automatically logged in as testuser@mystorespace.com). I really don't like having to register in order to try something, so I assume most users feel the same way.