WeblogToolsCollection.com has a two–part interview with Ian Stewart about changes in the way Child Themes are implemented in WordPress 2.7. WordPress already has a fairly powerful Parent-Child Theme setup that allows you to set up a Theme as a parent and then create a Child Theme that inherits everything in the child theme. You can then screw around with the CSS in the child theme to get the look and feel you want without touching the parent theme.
This way, you avoid having to redo all sorts of changes when the parent theme gets upgraded due to a new WordPress version, etc.
WordPress 2.7 kicks it up a notch,
Secondly, as of WordPress 2.7, template files located in your Child Theme folder will be used instead of the template file in your Parent Theme folder. Don’t like how the header is coded up for a particular theme you want to edit? Copy the header.php file from your Parent theme into your Child Theme folder and make the change there. WordPress, as of 2.7, will look for header.php (or any possible template file) in the Child Theme first and use the Child Theme template file instead. This new feature in WordPress 2.7 makes custom theming really exciting and even easier.
The first thing I always have to do with any WordPress theme is modify the header, and this will make it easy to do that just once and then not have to worry about upgrades and/or when I change Sandbox-based themes (copy the header.php to the new child theme directory).
Very nice.