[sparky2]

Slick hack, but I'd prefer a LINK ~~ rather than assuming each user wants to "view similar threads" every showthread.php page view.

Also, it seems like this hack is a LOT of work just to duplicate functionality that's already built into search.php
(select a subforum // search in titles only )

In showthread.php, you might want (need) to check $searchenabled and $cansearch values
and conditionally null the template if search isn't currently enabled...

but piping it to search.php would take care of:
-- PARENT/sibling title searches
-- permissions
-- runs the title text through BADWORDS

In other words, I'm suggesting that you try this approach:

Create a link to search.php (essentially, a submit button)
& pass post['title'] and forumid as hidden fields
(user sees nothing other than a button, or link, labeled "find similar threads")