[vedman]

Nevermind, I found it, finally. Of course, it was a tiny little thing, and to be honest I don't know if it was added to 3.5.1 for security reasons... or... er... hope it's not super important.

init.php, line 202:

PHP Code:

    if (PHP_VERSION < '5' OR ((SAPI_NAME == 'cgi' OR SAPI_NAME == 'cgi-fcgi') AND $vbulletin->options['gzipoutput'] AND strpos($_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS') !== false))
    {
        define('NOSHUTDOWNFUNC', true);
    }
    else
    {
        vB_Shutdown::add('exec_shut_down');
    } 


The php version check at the beginning of the IF statement was added in 3.5.1. That was it ...I'm running PHP 4.

I removed PHP_VERSION < '5' OR , and sessions are once again being recognized (rather, it now gets as far as the database session handling). Sweet.