Some of the tips for promoting new forums could probably be used to promote your forum software. For example, in forums that allow sigs, you could make helpful posts that include your URL and a tagline.
One way to use viral marketing is to be sure that all e-mails your forum sends (notifications, passwords, "mail this page", etc.) include the forum software name and URL used to generate the mail.
If you can find people who review software and blog their comments, try to get them to take a look. Good luck!