Multiple sites/domains with content all managed by single installation of WordPress

This should be doable.

First step would be pointing each domain to the same document root/install of WordPress.

Next up, use WP_HOME and WP_SITEURL in wp-config.php that change based on $_SERVER['HTTP_HOST']. Example:

<?php
define('WP_HOME', 'http://' . $_SERVER['HTTP_HOST']);
define('WP_SITEURL', 'http://' . $_SERVER['HTTP_HOST'] . '/wp');

That should force your permalinks (at least the ones generated dynamically), enqueues, etc to work correctly.

From there, I would create a metabox in the admin with some checkboxes for available domains. Each available post type should have this.

On the front end, hook into pre_get_posts, look for the current domain as the meta key, change the query to only fetch posts for that domain.

Quick example (untested, use with caution):

<?php
add_action('pre_get_posts', 'wpse70213_change_q');
function wpse70213_change_q($query)
{
    if(is_admin() || !$q->is_main_query())
        return;

    $key = 'wpse70213_host_'; // or whatever you want this to be
    $host = strtolower($_SERVER['HTTP_HOST']); // normalize http host

    // not sure if you can set meta_query like this...
    $q->set('meta_query', array(array(
        'key'   => $key . $host,
        'value' => 'on', // or whatever you use for "on" checkboxes
    )));
}

In short, it’s doable, but you’ll need to be in control of the entire WP installation.

Other things to consider:

• How do you deal with nav menus?
• What about sidebars?
• There will only be one, central taxonomy set for a given site — is that an issue?
• How much overhead does using meta_query on every page add?
• Users that don’t have any posts on one site will still have an author page on that site.
• You’d have to invent a user management system that mimics multisite’s assignment of users to blogs if you need functionality like that.

This is a really interesting idea. Might be good plugin material!