Child themes reference parent themes by directory name, and in a normal install all your themes live in wp-content/themes/
, so I’d say it’s fine to reference those themes by their relative path:
include '../parent-theme/some-file.php';
If that makes you uncomfortable, I observe the following constants in WordPress 3.0.1 with a twentyten child theme called tt-child:
TEMPLATEPATH /home/adam/public_html/wp3/wp-content/themes/twentyten
STYLESHEETPATH /home/adam/public_html/wp3/wp-content/themes/tt-child
So you can do the following in your child theme to reference the parent theme directory:
include TEMPLATEPATH . '/some-file.php';