The concert is by the electronica band Underworld. Apparently, news of the event has been kept quiet so as to not overwhelm the AT&T network. You can tune into the stream at 9pm PT.
HTTP Live Streaming broadcasts content in real time, though there can be a latency of up to 30 seconds. The content to be streamed is encoded into an MPEG transport stream and split into segments around ten seconds long. The protocol asks for the first couple clips then requests additional clips as needed.
The server can maintain multiple versions of the clips in different formats so a user with Wi-Fi would be broadcast a higher quality clip then someone on 3G or Edge. If the quality of your connection changes you are sent a corresponding higher or lower quality clip. This lets you experience the best quality video based on the bandwidth available.
Apple is using partners at Inlet Technologies and Akamai to power tonight's show.