It appears that if 14,000 people gather around in the same park, none of them gets a cellular signal. Probably because the cellular towers around suffers from congestion. Not being able to handle the load, the cellular operators actually lose money. I bet that many fans like myself wanted to Tweet/Buzz/LiveBlog/SMS/Video-chat/whatever during the concert, but couldn't.
I wonder whether mobile cell-towers exists, and if they do - why don't cellular operators station those near crowded places when needed.