This piece of code will do the trick. Place it in your functions.php or functionality plugin.
<?php
function my_single_template_by_post_id( $located_template ) {
return locate_template( array( sprintf( "single-%d.php", absint( get_the_ID() ) ), $located_template ) );
}
add_filter( 'single_template', 'my_single_template_by_post_id' );
Gist: https://gist.github.com/4184748
BTW, I don't think this should ever be implemented into core. There are a handful of filters out there that can help you target the right template file of your choice (the majority of WordPress users will simply never use this feature). And it hurts how easy it is to implement.