I just learned about a service from Weblog Tools Collection called Notifixious for allowing visitors to be notified via IM, email, or SMS when a new post is published to a blog.  This is a pretty slick tool that is easy to add to a WordPress blog and really simplifies these kinds of notifications.  The plugin provides a widget that can be added to a sidebar which allows the visitor to select how they want to be notified.  Clicking the Follow button sends them to the Notifixious web site to confirm their subscription.   I’ve added it to a couple of my sites.  We’ll see how well it works.