[Simon Lloyd]

You could do this:
In forum display find:

$headinclude
<title>$foruminfo[title_clean]<if condition="$pagenumber>1"> - <phrase 1="$pagenumber">$vbphrase[page_x]</phrase></if></title>
<if condition="$show['inlinemod']"><script type="text/javascript" src="clientscript/vbulletin_inlinemod.js?v=$vboptions[simpleversion]"></script></if>

and directly below add

<script type="text/javascript">

function show_confirm()
{
var r=confirm("You need to answer yes or no to this question");
if (r==true)
  {
    document.location.href="newthread.php?$session[sessionurl]do=newthread&f=$foruminfo[forumid]";
  }
else
  {
  document.location.href="index.php";
  }
}
</script>

then find

<if condition="$show['newthreadlink']"><a href="newthread.php?$session[sessionurl]do=newthread&amp;f=$foruminfo[forumid]" rel="nofollow"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" />

and replace it with

<if condition="$show['newthreadlink']"><a href="#" onclick="return show_confirm();"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" />

search for second instance of

<if condition="$show['newthreadlink']"><a href="newthread.php?$session[sessionurl]do=newthread&amp;f=$foruminfo[forumid]" rel="nofollow"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" />

which is below

<!-- controls below thread list -->

and again replace with

<if condition="$show['newthreadlink']"><a href="#" onclick="return show_confirm();"><img src="$stylevar[imgdir_button]/newthread.gif" alt="$vbphrase[post_new_thread]" border="0" />

now before starting a new thread they will be asked whatever question you set but the buttons cannot be changed as they are for a html alertbox.