I don't believe this has been mentioned before, but PHPList would seem to be a good fit for this feature. It is a lightweight mail list program that requires PHP and MySql. It also features an RSS to email list function that might also be nice when combined w/ mediawiki's RSS feed features. Here is a link to a NewsForge article about PHPList:
http://software.newsforge.com/software/04/08/20/1538204.shtml?tid=79&tid...