I was chilling out, thinking about going to sleep when the secret mail sent me this link to Om's post on Twttr
There's lots of ways to describe twttr, but at its simplest, it is a service that notifies your friends (people that care) about your presence and status. Technically, it's very simple: it sends your SMS text messages to all your friends' phones. On the flipside, you will receive txt msgs from those friends that you want to get msgs from. You can view the stream of text messages on the web, from yourself or all your friends. While this has the potential to spam your phone, I think they're on the right track with development on mobile devices.
Thought of another way, it's a more fine-grained blog that encourages short, and otherwise ethereal thoughts to be sent. Where as IM is presence on a computer or sidekick, and blogging is presence in a world of ideas, twttr encourages the idea of presence as "what are you doing at this moment" to bleed into real life.
Mobile devices have certain chacteristics unique to the platform. I think this application takes advantage of the unique characteristics of a mobile platform, unlike the boring applications that Verizon comes up with, like directions and video casting.