To demonstrate a pair of their products, Google arranged for a group of skydivers to jump out of a blimp and parachute onto the roof of the Moscone Center in San Francisco, the building in which the Google I/O was being held. The divers were each wearing a pair of Google Glass networking glasses and video chatting on a Google+ Hangout.
Here's what it looked like from the ground:
I think this is what Robin Sloan was referencing in his tweet earlier:
Watched #GoogleIO. This company is totally Doc Brown. In one corner, an automatic banana-peeler; in the other, A WORKING TIME MACHINE.