Use theme constants in plugin?

TEMPLATEPATH and STYLESHEETPATH will be deprecated in the near future. It is not safe to rely on these constants. Use get_template_directory() and get_stylesheet_directory() instead.
And wait until the init hook (or after_setup_theme before you use them to be sure all needed files are loaded.

Example for a plugin:

add_action( 'after_setup_theme', 'wpse_50359_set_plugin_basics' );

function wpse_50359_set_plugin_basics()
{
    if ( is_child_theme() )
    {
        // do something awesome …
    }
}