<p>I am looking for a way to maintain PHP sessions across multiple domains on the same server. I am going to be integrating my sites with a Simple Machines Forum so I will need to use MySQL based sessions. Thanks!</p>

Depending upon your preferred method of modifying PHP variables (Apache's config, .htaccess), change the session.cookie_domain value to be a consistent value.
I have multiple sub-domains, and each VirtualHost section in the Apache config file contains the following line:
php_value session.cookie_domain  mydomain.com
The syntax should be similar if you make the changes in a .htaccess file.
Updated for bobert5064's comment:
For multiple domains (ie domain1.com, domain2.org), I think it is only necessary to choose a common domain name (ie domain1.com).  I have never tried this, so I cannot verify that it works, but the logic seems accurate.
There is also a method to set the variables direction in PHP described at
http://us.php.net/manual/en/function.session-set-cookie-params.php
.  The documentation makes no reference to the ability or inability to set cookies on a different domain.