As I commented here, I've found some little problems in this plugin (no more maintained and supported since 2007-12-14), and due to the lack of similar plugins, I've decided to write a new release 2.1.3, with some patches
Version 2.1.3 changelogs
1) fixed localization problem (localization didn't work for me in WP 2.8.4)
2) created Italian po and mo files for Italian transalation
3) Major modification in the "theme embedding capabilities" of Subscription manager
3a) Modified admin page, no longer takes path to header, sidebar, footer files. It asks for (X)Html code to insert after header, befor sidebar(s) and after sidebar(s) (many thanks goes to Customize your community plugin for the original idea!)
3b) One or more sidebars supported
Feel free to download this 'new release' from there.
Many thanks go obviously to the original author Mark Jaquith.