I just invented that trend. Hang on and I'll explain it. First, more on Pocket Doorbell.
This little app is a button on your iPhone that you press when you arrive at a friend's house and want them to come open the door for you. Instead of pressing the doorbell, and disturbing the whole house or apartment, and instead of bothering to look up their name on your phone to call them (or having Siri do it for you), you just press the button. The app knows the location of who you're visiting thanks to the phone's GPS; if you don't have your friend's address in your contact list, you can select their name the first time you visit and from that point on Pocket Doorbell will match the location to the name.
The app will either send your friend a quick SMS ("I'm here!") or call them for you.
Simple. And, yes, rather silly on the surface. But actually, as I said, not a bad idea. It does what a doorbell does but with higher precision. Like a doorbell, it tells the friend you are right outside. Unlike a doorbell, it's pretty much guaranteed to reach a modern person (that is, one whose phone is never more than an arm's reach away), no matter where in the house or backyard they are, or how noisy the party you're going to. It will also tell the person in the house who is outside, waiting for them. It's like Caller ID for your door. And if your friend isn't at home, well, now they'll know you're there and they can communicate with you.
Yes, you can do all this by just looking them up in your contact list and calling them. That's why I had to invent a major trend to justify writing about this app.
Pocket Doorbell is yet another case where a smart, mobile appliance can do a simple job better than the old-fashioned mechanical device it replaces.
Here are some other appliances that work better when virtualized to a mobile device.
The home phone. Obviously.
The thermostat. A smartphone-powered control panel for a home thermostat lets you control your heating appliance from the couch, but that's not all. It can automatically adjust temperature for you when come home or leave, and you can use it to pre-warm or -cool your house if you're coming home at an unusual time. See the Nest.
The alarm system. All home alarm systems should be connected to a smartphone app (and many are). The whole idea of an alarm, after all, is that it's an appliance that should be able to tell you what's happening in your house when you're not there. A mobile app is a lot more effective than a siren for that (although there's still a lot to be said for a piercing alarm when it comes to alerting burglars that you're on to them).
The remote control. A smartphone remote can know so much more about you than even the smartest programmable Harmony. The trend here is not just the smartphone (or tablet) remote, but rather using the mobile device as a media collector that shoots content to the entertainment system (see: Airplay). Most smartphone remotes are awful so far, but there's a lot of room to improve the living room entertainment experience by using a smart, capable, connected device instead of a brick full of buttons.
The key. The great thing about electronic keys is that you don't have to give a friend or service person a full pass to your house with one, and you don't have to worry about an electronic pass being lost. You can just configure access for when you want. And like modern electronic touchless car keys, you could also have nice hands-free unlockeers for when you're at your door with an armful of groceries.
The light switch. Ok, that's just for geeks. For now.
Finally, although it's not a home technology: the highway sign. A smartphone navigation device makes the paper map and the expensive street and highway sign obsolete. You get the data you need when you need it: It's precise and targeted. I'm not suggesting that highway bureaus and municipalities stop maintaining signs, but if you navigate by GPS instead of by paper map, you have the superior replacement sitting on the dash (preferably in a good car mount, for safety).
Obviously, there are reliability, privacy, and safety issues with all of these concepts, to varying degrees. And for the record, I'm a big fan of stupid mechanical devices that don't rely on electricity or data networks being up all the time. But even simple things can be made smarter and more useful. Pocket Doorbell and Nest are just the beginning.