Modified version of Mark's class that includes a flush() member function.

gistfile1.php

<?php
/*
Usage:
	$frag = new CWS_Fragment_Cache( 'unique-key', 3600 ); // Second param is TTL
	if ( !$frag->output() ) { // NOTE, testing for a return of false
		functions_that_do_stuff_live();
		these_should_echo();
		// IMPORTANT
		$frag->store();
		// YOU CANNOT FORGET THIS. If you do, the site will break.
	}
*/

class CWS_Fragment_Cache {
	const GROUP = 'cws-fragments';
	var $key;
	var $ttl;

	public function __construct( $key, $ttl ) {
		$this->key = $key;
		$this->ttl = $ttl;
	}

	public function output() {
		$output = wp_cache_get( $this->key, self::GROUP );
		if ( !empty( $output ) ) {
			// It was in the cache
			echo $output;
			return true;
		} else {
			ob_start();
			return false;
		}
	}

	public function flush() {
		wp_cache_delete( $this->key, self::GROUP );
	}

	public function store() {
		$output = ob_get_flush(); // Flushes the buffers
		wp_cache_add( $this->key, $output, self::GROUP, $this->ttl );
	}
}