I don't know if you've noticed, but in the right column of this blog I've added a small little note area called 'What I'm doing". It's driven through an online service called Twitter who's only function, at this point, is to tell other people what you're doing. Groups can be formed on Twitter's web site so you can check out what all of your friends are doing.
Twitter is currently being used by bloggers exactly like I am. The service is very easy to use, updates can come through an SMS message to a fixed number, updating your status through a form online or by instant messaging Twitter through any of four services. All you do is type in what you're doing in the message and send. Setup is a snap and updates are near instantaneous.
For developers, the Twitter API is completely open and flexible enough to leverage their messaging framework to feed in updates to any number of applications. While the primary use is personal, as I said before, more interest has surrounded Twitter's possible corporate uses. Liz Gannes at GigaOM has a nice post about this very topic.
Corporate examples could be allowing a customer service team to update system status in real time, pushing offers and specials on an e-commerce site quickly or recording billable time entries for later invoicing (the proprietary nature of this last point is a concern with the current hosted model of Twitter). As with any service like this the business case needs to be in place to make this a strategic benefit to the end user as well as the people who use it. It is time consuming, even if it just takes a couple of minutes, and you have to remember to do it. If, however, you're looking for an existing messaging framework which handle multi-mode input through existing device platforms, and you foster a community that cares what its members are doing right now, something like Twitter could be for you.
If you're wanting some type of big brother, what-is-Ted-from-accounting-doing-right-now, application either enable GPS tracking on their phones or implant RFID tags. Both options are more than a little creepy.