Interesting plugin, which I spotted at wpmu btw :)
One question: Does it support multisite?
How about sending mail only to the (multiple) blog administrators, ie. excluding editors, subscribers etc from the receivers. We have a lot of blogs that are inactive but we don't want to remove the users so we've demoted them to suscribers. They don't need our newsletters, but the blog admins do.
If these two issues are addressed now or in the future versions, I commend you on making an excellent tool!