If you aren't sure what App.Net or ADN is, don't be surprised. It's a still fairly underground social network modeled after Twitter, but without the restrictions Twitter has started putting in place. The folks at Tapbots seem to be fans of the new network and have re-purposed Tweetbot for ADN. It's $4.99 for the iPhone/iPod touch version and another $4.99 for the iPad version.
This might provide a nice boost for ADN, but it's still a barely known service mainly filled with super-early adopters who cross-post to every service under the sun and those with a grudge against Twitter. Hopefully that will change.