Locations and times were posted on websites and on facebook for people who were interested in participating.
The group organizing the flash mobs, MedMob, explains their motives on their website:
Our intention is to create an environment for people from all walks of life to come together in meditation. By exposing people to meditation through public display of meditation and sending positive intentions out into the world we can lead by example. Simple acts can stimulate major paradigm shifts in thinking.
The organization has organized flash mobs before, and from comments posted yesterday and today on facebook, it seems possible another will happen in the near future. For more information, visit their facebook page.
A few weeks ago there was a similar flash mob event organized by Black Swan Yoga in Austin, TX. Here's some video coverage of that event: