If you wanted to, you could use existing forum software and/or Wordpress to facilitate what you want, which would be easier than building your own forum from scratch. You could, with that existing framework, set up your own little API to communicate from the iPhone app to the server- for example, send a $_GET request to a PHP script on your server, which would return a list of forum topics. You could have similar PHP scripts that could do similiar functions, like adding a post or deleting topics.
That's pretty much how I've got it set up on an iPhone app I recently made- my server has a basic forum system, and I just wrote a couple of PHP scripts to return information from a MySQL server. However, if you'd particularly prefer to use Wordpress/Amazon S3/whatever else, then I could give more specific instructions relating to those services.
*EDIT*
Here's an example PHP script you could use (after you've created databases):
forumcategories.php
<?php
// insert database connecting logic here
$query = mysql_query("SELECT * from categories");
echo "<categories">;
while($row=mysql_fetch_array($query)){
echo "<category><id>" . $row['id'] . "</id><title>" . $row['title']; . "</title></category>;"
}
echo "</categories>";
?>
This is a really simple script- of course, you would need to add in code to connect to the database (which can be found easily online) and probably some error checking, but other than that, it will do the trick. What you would do in the iPhone app is send a request to http://yourserver/forumcategories.php and it would return XML listing all of the categories, which can easily be parsed with NSXMLParser and placed into a UITableView, for example.