Three of the hottest topics these days in the tech world are Twitter, mobile social networking, and the iPhone. For those who don't know a thing about these, let me take you on a quick overview.
Twitter is a microblogging network that allows you to blog in spurts of 140 characters or less, which allows you to blog via text message on your cell phone. You can also follow your friends and "tweet" at them, thereby holding conversations via blog. The big controversy right now is whether it will make the jump from the early adopters (tech people use it so incessantly that one of the hottest areas is building applications around Twitter and Facebook feeds) to the mainstream. The issue is that to get value from it, you need a lot of your friends to use it as well. But once you're all on it, it's horrendously addictive, to a point where the tech world has a crisis every time the servers go down (which is too often).
Mobile social networking is the movement to add physical location relevance to the functionality of social networks (Facebook, MySpace, etc--though most talk on the social network scene is separate from the big guns in traditional social networks).
The iPhone has two big things coming soon: a developers platform (allowing anyone to create their own applications for the iPhone) was recently released, and those programs will be released soon; and there are loads of rumors about a 3G version (meaning faster data transfer for internet, plus potentially really good GPS) of the iPhone being released within a month, potentially with a price drop to $199.
Up til now, though, these conversations have been largely separate from one another. There are exceptions--TechCrunch wishes there were an iPhone-only social network, Twitter essentially acts as a stripped down mobile social network--but essentially these topics are discussed in their own realms. I take issue with that.
If I went into all the possible tie-ins and reasons, I'd be writing pages upon pages. Instead, let me posit my idea of a "killer app."
Picture an iPhone/mobile application that keeps track of where you are and adjusts accordingly. You can see where friends are and message them directly, or you can "tweet" (to borrow the Twitter term) from/about where you are. When you tweet, your message gets geotagged. All the public messages from that immediate area (adjustable on preference) would be accessible to anyone in that immediate area or to anyone searching for that area (thinking they might go there). Think of it as dynamic geospecific message boards.
If that doesn't immediately make sense, consider how Twitter first became popular: South By Southwest. People twittered about where the hot parties and shows were, other people picked up on those tweets and showed up to join the fun. Now translate that to everywhere.
You've got a few minutes away from a conference in another city and aren't quite sure what to do. Whip out your iPhone, launch the program, see what others in the area are up to, see if any friends (or friends of friends) are around, get geotagged Yelp reviews, etc. In the most active areas, you could even post a sort of "help me" question and anyone within your range could see any help out. You could also find a way to reward folks who were willing to receive and respond to TXT messages when those "help me" messages popped up near them.
There are, of course, also tie ins to media, being able to tweet photos and videos from anywhere (I would think someone will be making an app that lets you Twitter and automatically upload any attached video or photos to YouTube/Flickr). Fuzz just came out with a Twitter-like mashup with Seeqpod and Skreemr called "Blip" that lets you attach streaming audio to your tweets (follow me here: http://www.fuzz.com/blip/user/tygergolf), and I can only imagine that more will follow.
Personally, I struggle with Twitter simply because so few people I know are on it (if you are, hit me up at http://twitter.com/tygergolf). I think the way to bridge that gap is to build utility around it that makes it useful both to those who don't use it, and to those who are interested in using it, but don't have friends on it. The geotagging ability of the iPhone makes that possible, and can essentially turn it into a social network. I'm not the only one who sees this, but I think there are a number of ways to go about all this integration, and there will be a long tail with this sort of application if they are open enough.