PubSubHubbub. Say that three times fast. If you're not familiar with PubSubHubbub, it's an extension of the RSS and Atom protocol that allows the feed to push content out when it's updated. That's just the sort of technology we should see from check-in APIs like Foursquare and Gowalla.
Update: As folks have pointed out in the comments, Gowalla supports PubSubHubbub.
The idea is that feeds can tell the apps when they have new content to check for, rather than having the apps check at regularly scheduled intervals. Pretty cool, right? Well what if we took the concept beyond just updating other reader apps when there was stuff to check for and thought about check-in services, in particular. What if your Foursquare or Gowalla check-in automatically triggered another application to check your feed for your latest check-in and do something? You could easily create apps that instantly talk to one another without having to build some sort of push notification system on your own.
In the video below, recorded at GlueCon, Twilio's Jeff Lindsay makes a case for these sorts of real-time APIs:
Foursquare developers are asking for it. Honestly, I'd love for someone to take this concept and build an app that checked me in to Foursquare when I checked in to Gowalla, like a passive single check-in service. PubSubHubbub could be the end-all be-all cure for 'check-in fatigue'. But it could go further, like a map that automatically updates when my friends check in.
Of course, all this is theory until Foursquare or Gowalla implements PubSubHubbub in their user feeds. Currently, not many popular applications make use of PubSubHubbub, which is a shame, considering how powerful this would make RSS feeds and RSS-based apps. Check out the PubSubHubbub Google Code page. It's pretty interesting stuff and it would be great to see it become more prevalent.