functions.php

<?php

add_filter( 'posts_request', 'pre_db_hit' );
function pre_db_hit( $request )
{
	global $diy_cache_current_request;
	$diy_cache_current_request = $request;

	// use request string to see if expired...

	if ( $diy_cache_expired )
		return $request;  // Let query happen
	return null;  // null request shuts down execution of the query
}

add_filter( 'posts_results', 'post_db_hit' );
function post_db_hit( $post_objects )
{
	global $diy_cache_current_request;

	// use request string to see if expired...

	if ( $diy_cache_expired )
	{
		// Store the post objects in the cache
		// under a key consisting of a hash of
		// the request string...
	} else {
		// Get the post objects from your DIY
		// cache and return them...
		return $cached_post_objects;
	}
	return $post_objects;
}