How to exit a plugin’s execution mid-stream?

A possible solution would be to hook in way late (template redirect, for example), do your conditional check. If it passes, include (or require) a file that contains the stuff needed.

<?php
add_action( 'template_redirect', 'wpse30291_template_redirect' );
function wpse30291_template_redirect()
{
    if( ! is_singular() ) return;
    require_once( 'your-file.php' );
}

This is not going to work if your-file.php contains functions hooked into things that happened earlier (eg. init or wp). But if your-file.php contains things that, say, modified content (hooking in the_title or the_content, for instance) then it would be fine.