You do everything else with twitter now, so why not track your packages? I think this is a great idea for people who love twitter, and may be a reason for those that don't to get into it:
To use it:
1. Follow packagetrack on Twitter (http://twitter.com/packagetrack) and within a few minutes packagetrack will begin following you.
2. You can now Direct Message packagetrack with the nickname and tracking code, separated by a ":", eg. "Amazon Books: 23345631243" of a package you would like to track. Package Track will continually check your package and let you know when your package's status changes.
Thanks to Ren Provey of signalengine.com for this great twitter tool!