[wp-hackers] Re: Good practice for customizations to a theme?

[Eric Marden]

And beyond child themes, Thematic is more of a framework, too. There  
is a 'clean' way to develop with it, so that the core theme can be  
left intact and provide you an upgrade path with the theme, and keep  
your customizations separated, much like you're doing with your plugin  
code. The same methodology applies here too.

- Eric Marden
http://xentek.net