Ping.fm is an interesting service that lets you update your social networks simultaneously. You set up your social networks, such as Plaxo, Blogger, Twitter, Linked-in (30+ sites are supported), and then post to them in various ways. You can post from your ping.fm dashboard page, or via email or chat client. That said, I know I would be kind of irritated to get the same exact post from a person on multiple networks.
Don't Inundate Your Readers
I would say it's important to use a powerful service like ping.fm with care, and in a very specific way. Currently, I have the special email address from ping.fm in my address book, and I am updating only business-related sites like Linked-In with a status related to my professional situation.
UPDATE - Ping.fm was not working for Plaxo updates up until 30 Dec 2008, but now it is fixed.