The following method is not officially recommended. But if you understand what you are doing and what the consequences are, the benefits can outweigh the drawbacks.
See Share tables across instances (not recommended)
I'm using this setup to run a network of sites for a client of mine where content is identical across sites, but the presentation (including views, modules, variables, block placement) is different. My particular setup is Drupal 6, but it could easily be adapted for Drupal 7.
The first step is to configure each of the sites to run off the same database (using table prefixing) and codebase (using Drupal's multisite functionality)(*). Once the sites are all running off the same DB and codebase, you can create a $db_prefix array in your settings file.
In my setup, each of the sites shares all tables with the default site by default:
$db_prefix['default'] = 'default_';
And the following tables are unique per site (again, this is D6, D7 would be slightly different):
$db_prefix = array(
"sessions" => "site1_",
"authmap" => "site1_",
"variable" => "site1_",
"system" => "site1_",
"role" => "site1_",
"menu_custom" => "site1_",
"menu_links" => "site1_",
"menu_router" => "site1_",
"cache" => "site1_",
"cache_block" => "site1_",
"cache_form" => "site1_",
"cache_menu" => "site1_",
"cache_page" => "site1_",
"cache_update" => "site1_",
"cache_views" => "site1_",
"cache_views_data" => "site1_",
"views_display" => "site1_",
"views_object_cache" => "site1_",
"views_view" => "site1_",
"blocks" => "site1_",
"blocks_roles" => "site1_",
);
I've been running these sites in production for a couple of years now. Editors only login to the "default" site (defined in /sites/all) and all content (nodes) is managed there. Changes to nodes take effect on all of the "slave" sites immediately and without any intervention (save for whatever caching you have in place).
This is definitely an advanced option and it has it's drawbacks (upgrading between major versions of Drupal is nearly impossible), but if you are comfortable and understand what you are doing, it can be a great solution.
(*) As long as all of the sites are on the same server or have access to the same database server, I guess you don't need to set it up as a multisite. But to make sure there are no differences in core files, I would still recommend it.