This is an app called Street Orchestra, I think it is going to be quite popular. The video above shows what happens when the Gothenburg Symphony Orchestra tested the app, using it instead of their own instruments. (More video after the jump).
Conceived of by Swedish advertising agency SCP, and developed by HiQ the app is like a simple version of Rock Band that lets you play classical music with up to 200,000 synched iPhones. And it is free.
All your iPhone needs for you to join in is an internet connection over 3G