I’ve eventually updated Welcome Pack for WordPress 3.1. It should become available on wordpress.org shortly.
Welcome Pack is a BuddyPress plugin that enhances the new user experience. When a user registers on your site, Welcome Pack lets you automatically send them a friend or group invitation, a Welcome Message and can redirect them to a Start Page. You can also customise the default emails sent by BuddyPress to ensure that they match the brand and tone of your site.