$_SERVER['document_root'] will refer back to the document root of the server and not virtual host in which the PHP script is running. I'm not sure how you have your hosts set up, but for subdomains you'd normally have something like this in your Apache configuration:
PHP Code:
ServerRoot "/path/to/server/root"
DocumentRoot "/path/to/document/root" <!-- $_SERVER['DOCUMENT_ROOT'] should point here -->
NameVirtualHost *:80
<VirtualHost *:80>
ServerName "neskaya.net"
DocumentRoot "what/ever" <!-- Not here -->
...
</VirtualHost>
<VirtualHost *:80>
ServerName "forums.neskaya.net"
DocumentRoot "what/ever/lol" <!-- or here :p -->
...
</VirtualHost>
Also not that $_SERVER['document_root'] != $_SERVER['DOCUMENT_ROOT']. 
If you just use that variable as your absolute path, you never need to worry about the location of the script you're executing.
PHP Code:
//So if the document root of your server is one directory above neskaya.net:
include $_SERVER['DOCUMENT_ROOT'] ."/neskaya.net/include.php";
//Or perhaps if your document root is the neskaya.net folder:
include($_SERVER['DOCUMENT_ROOT'] ."/include.php");
//If you need to find out what that document root is:
echo $_SERVER['DOCUMENT_ROOT'];