This requires some pre-planning and organization by a central person. This is planned ahead of time and is typically not a big as an atomized flash mob or an interactive flash mob. These tend to be led by a comedy/art group.
"Glee" Flash Mobs
http://www.youtube.com/watch?v=XOiJkx4NtOM&feature=related
This a dancing one, I cannot tell if they are singing as well with the music, but if they are then it fits in with how I wanted my original idea. They are dancing to a mash-up of multiple songs, some parts choreographed, some not.
One Night of Fire
http://gothamist.com/attachments/garth/2006_07_30_onenightoffire.jpg
A bunch of people meet in Brooklyn where there are smaller things such as the one above. It is supposed to be a night of 'illusion' as the site for Danger says on its video for Past Exploits>Street
MP3 Experiment
http://www.youtube.com/watch?v=ZG7F9G4AEak&feature=player_embedded
They use an MP3 file to give instructions to perform certain acts at about the same time. The result is a really pretty lights show in NYC. This is actually a really good way to use the technology availible today, and an easy way to make sure people do not forget.