You could extract all the email addresses from the database with a tool like phpMyAdmin. I don’t know of a plugin that does this. If it’s a onetime thing, I would just grab the email addresses from the database with a query like:
SELECT DISTINCT user_email
FROM bb_users
ORDER BY user_email ASC
With all the email addresses, then just send out a mass email with your email client putting their email addresses in the BCC field.
Are you looking to do this regularly, and are you thinking this should be a plugin? I guess you’d need to know if the installation were integrated or not, then query the correct table, and also if it is an integrated installation, you’d need to separate out the bbPress registrations from the WordPress registrations.
There are some decent programs for legal mass mailing that you can use. For Mac, there’s a great program called MailDrop that will import email addresses from a spreadsheet. The email service, Constant Contact, does the same thing. Just export from PHPMyAdmin and import the info into your bulk mail program.
A plug-in would be easier, that’s for sure!