Okay, so nothing's been announced for a long time (though there are some snazzy articles that should be coming your way in the next month or two), but to remedy that, Cooba has initiated the JCS Collaboration Contest over at the JCF:
That's right, a contest not made for individual level designers. There hasn't been exactly a flood of collaboratively made levels (I recall seeing as many as two, at all), which is a pity because collabs sound like a bucket of fun to make, and they're interesting from an analyser point of view, too. Plus, more collabs in general is always a nice thing.
There are a lot more details, all of which can be found in the opening post in the linked-to thread. Basically you have until December 30th, must work with a partner, and make a changelog so Cooba can know that you've been properly collaborating. Go to it!
Added on: Friday, November 23 4.02 pm. Made by Violet CLM.