I could create a Bukkit modification to dump the online users to a text file, or submit it to a PHP page (then having it updated in the database, etcetera.) and fetched by a script / widget inside the forum.
IE: Bukkit modification tells a page (mcs.php) on the website here (via a POST request) that <server id> has <x> players online, and send the names of the connected players as well.
Then a script on the site here (the mcs.php) can save the information to a database table, and a widget inside the forum can query that information, IE: Get a list of people on X server, get a list of people on all servers, find what server Y is on, stuff like that.