[Gearloose]

A simple (=DUMMY!!! but fully working!) way to do this is:

In printthread.php find:

PHP Code:

while ($post = $DB_site->fetch_array($posts))
{ 


Add below:

PHP Code:

if (is_null($_REQUEST['p']) || ($post['postid'] == $_REQUEST['p'])){ 


Find:

PHP Code:

    eval('$postbits .= "' . fetch_template('printthreadbit') . '";'); 


Add below:

PHP Code:

} 


Then you have to add a link to trigger this. You can add it anywhere you like in the postbit but I'd suggest here:
In the postbit (or postbit_legacy) template find this:

<if condition="$show['postcount']">#<a href="showpost.php?$session[sessionurl]p=$post[postid]&amp;postcount=$post[postcount]" target="new"><strong>$post[postcount]</strong></a></if>

Add below:

(<a href="printthread.php?$session[sessionurl]t=$post[threadid]&amp;p=$post[postid]" target="_blank">Print</a>)

You could replace the "Print" text with an image if you prefer.

Note:
This is NOT to considered as a real hack but just as a simple quick solution to accomplish what you asked for.
It just vanishes the rest of the posts but the one you want to print, but still the whole of the printthread.php code is executed and all of the queries also, so it's a waste of resources. If you don't mind for this waste, then it should fit you.
(If there is interest in deploying a more decent version of a dedicated printpost.php script, I could do it fairly easy I guess. If so, I may come up again when I find the time and if noone else do it until then.)