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Until recently, iPhone location app designs were limited by the constraints of single-tasked OS capabilities: launch Foursquare and check-in, use Yelp find a nearby place of interest, open another app to update your status and tag it with location.

All of these scenarios require users to have a participatory role in publishing and sharing. This works well for some apps that require active engagement such as broadcasting your Twitter status and where you'll be later. But an entirely new class of compelling scenarios really shine when you use a technology called geofencing to leverage background location and push location updates up to public and private clouds.

The Hotlist is going mobile. The service, a trend-tracker that connects users with upcoming events in the area and across their social networks, is now available as an iPhone application.

Although location-based social networking (LBS) apps like Foursquare, Gowalla, MyTown, Brightkite and Loopt may be all the rage right now - in terms of media hype at least - reports have shown that only a relatively small handful of early adopters are actively using these apps on a regular basis (a few hundred thousand up to 4 million, depending on the service).

Instead of tapping into the niche market of LBS users to find local hotspots, The Hotlist figured out a different way to surface these so-called "geo-trends": by tapping into Facebook and Twitter instead. The service analyzes public events and other data across social networks in order to offer a personalized "things to do" application. But up until today, that app was Web-only.

Google has begun opening up access to a new Application Programming Interface (API) called the Places API. Developers building apps that include a "check in at this place" feature can use the Places API to search across all the places users might check in for basic information like business name, address, phone number and other descriptive information. That information will be editable by the businesses listed and no caching of data is allowed, so apps will have to ping Places regularly for real-time data.

Making this data as free and easy to use as Google Maps is today could create a foundation for new location-savvy apps to bloom throughout the mobile web, with far less overhead than such apps have to wrestle with today in order to provide a rich user experience. One catch? All these apps will have to be integrated with Google's Adsense.

Technology entrepreneur and Dallas Mavericks owner Mark Cuban has just invested in a video company that can identify how many people are in a specific location at a particular time. While currently the technology is used for analyzing "traffic patterns" and "security," writes Cuban on a blog post announcing the deal, the next step is adding facial recognition software for the purpose of check-ins using location-based software, he says.

"Rather than someone checking into a specific application, we would already know you are there," Cuban explains. That prospect is either incredible or downright scary, depending on how you feel about privacy.

With a number of location-based social networking applications currently available, and no clear winner as of yet, a micro-industry for universal check-in applications has sprung up, with several mobile apps available for the early adopter set. We've looked at a couple of these in the past, including the mobile Web application developed by the Brighkite team, Check.in, and GeoLorean, a check-in aggregator that features Check.in's functionality.

Today, we're experimenting with a third: the oddly named, but feature-rich "Footfeed."

Do you want to chat with friends who are watching the same TV shows you are or reading the same books? Do you want to check in to record and share a history of your offline entertainment activities? Many startups think you will and the number of apps for that is growing fast.

The latest entry into the field is from GetGlue, a startup that offers a popular plugin for social Web browsing, that just announced its iPhone app for entertainment check-ins today. The service stands out in the field as one of the only offerings that features books and other activities, and it has an excellent recommendation feature that's sure to be adopted by competitors in time. What are other startups doing in this market? Below we offer a feature comparison between GetGlue and competitors HotPotato, Kickfour, Tunerfish, Miso and Philo.