Creating a default template for WordPress custom post types with Theme override. This is broken down to limit it to a single view, but could also be modified for a default archive view, etc...

gistfile1.php

<?php
add_filter( 'template_include', 'default_mycpt_template', 100 );

/**
 * Setup a default template for your CPT if the current theme doesn't already have one.
 * @return void
 */

function default_mycpt_template( $template )
{
    $post_type = 'my_custom_post_type';

    // Check if the current theme has a template for our post type.
    if ( FALSE !== strpos( $template, "/single-$post_type.php" )
    {
        return $template;
    }

    // Send our plugins version of the post type layout
    if ( is_singular() && $post_type === get_post_type( $GLOBALS['post'] )
    {
        return dirname( __FILE__ ) . "/single-$post_type.php";
    }

    // Not our post type single view.
    return $template;
}