[TECK]

Ok, another idea I came up with... If the crawler is Google, for example, and it's hitting the actual forum pages, not the archive, you could use this trick to remove the sessionhash from the templates:
FILE: functions.php
ADD ABOVE [high]?>[/high] THIS CODE:

PHP Code:

function no_sessionhash()
{
  global $session;

  $agent = array(
    'crawl',
    'googlebot',
    'gulliver',
    'ia_archiver',
    'internetseer',
    'linkalarm',
    'mercator',
    'openbot',
    'pingalink',
    'psbot',
    'scooter',
    'slurp',
    'slysearch',
    'zeus',
    'zyborg',
    'otheruseragentcrawleryouwant'
  );

  foreach( $agent as $useragent )
  {
    if ( stristr( getenv( 'HTTP_USER_AGENT' ) , $useragent ) )
    {
      $session['sessionhash'] = '';
    }
  }
} 


FILE: /forum global.php
ADD ABOVE [high]$header='';[/high] THIS CODE:

PHP Code:

no_sessionhash(); 


The list above will cover all major crawlers.
Make sure you enter the actual AGENT name ONLY.
To test it, enter for example:
'mozilla',

It will remove all your session hashes, if you browse with Internet Explorer for example.