I'm sure this has been done, so let me build on what must be an obvious idea: a plugin that will post a tweet after every new user signup / blog on a WPMU install. the plugin would only post the tweet once the new user has logged in and edited / posted their first item.
it would also be amazing if the plugin were to grab the first tags from the user's intial post and used those as hashtags:
"John has a new [sitename here] blog: john.website.com #wordpress #tags #stupidity"
so far i'm using a long-way-around:
http://www.blogcastor.com/automatic-follow-up-emails-for-new-users-plugin/
customise the output to wait 1 day before sending a 'follow up email' to my account at > http://www.tweetbymail.com/
