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.
With the real-time web being claimed by some to be one of the core components of Web 3.0 it's unsurprising that we are seeing more real-time APIs and real-time mashups being developed. One mashup has just been created using Kwwika and Superfeedr to demonstrate how two real-time platforms can be integrated to create a real-time news reader application.