Edit: Oh, this extension is for PunBB 1.3. But I think it's possible to make a mod for 1.2 based on it.

If your host allows it,

exec('ping '.$ip);

But unless you have a dedicated server, I doubt it
Haven't used this, but maybe it works: http://be2.php.net/manual/en/function.imap-ping.php

No ping services like ping-o-matic, I'm looking for  a such thing too.
It lacks of a nice plugin for that ;'(

exec('ping '.$ip);

But unless you have a dedicated server, I doubt it
Haven't used this, but maybe it works: http://be2.php.net/manual/en/function.imap-ping.php

Is there any way a "ping" could be generated everytime there is a new post?

Thank you.