I’m an on and off again Foursquare user. I didn’t fully “get it” until I was travelling and needed to find a place to eat. It was then I realized this is more than just announcing you’re somewhere – it’s a discovery tool. Over the last bit they’ve been slowly migrating all of their maps to OpenStreetMap from Google Maps.
Now they’ve announced you can actually add data to OSM – they’re encouraging it. Right now if you’re in the UK, Australia, Germany, or Brazil you can edit OSM data (I assume from foursquare). I’m guessing you’ll be encouraged to add points much like the cloudmade app (which is now D-E D Dead)  and the fulcrum app (which is awesome if I had an iphone).
This is why Google maps is going to end up second in the maps race. Yes – Google Maps/Earth is flashier – but you can’t be keep up if it keeps getting easier and easier to contribute to OSM. I guess we need ESRI to scream “but but but authoritative” or “VGI” just to round out the discussion.
So go fourth and eat and map – you can now almost do both at once now. You might end up with a raging case of “VGI” if you’re not careful.
**EDIT**
Just got a comment and to make sure you see it I’m reposting it here – from Harry Wood
I think it’s not quite as happily tied together as you’re imagining.
The OSM editing features are just about editing features seen in the foursquare base-map. They want to see more streets sketched in from imagery, and more names on streets in cities in Brazil for example. When you add a new restaurant to foursquare, that doesn’t go into OpenStreetMap or vice versa. They’re still running a system which is building a proprietary crowd-sourced POI database, only accessible piecemeal by API.
Also these new OSM editing features are not shown to all 15million foursquare users. It’s only for “super usersâ€. Folks who have dedicated a lot of effort to become foursquare super-users, by doing things like correcting locations of bars & cafes. Even so, this is great news. Some level of integration with OpenStreetMap is better than just viewing our maps. And with these super users now directed to make improvements to the base-map (OpenStreetMap), we can expect some of them to take a deeper interest in our project.
I haven’t actually seen the features. I guess it’s an embedded instance of the iD editor on the foursquare website, and maybe they have disabled display & editing of the OpenStreetMap POIs data in order to avoid confusion (but this is just guesswork).
