The attacks were executed with the Low-Orbit Ion Cannon (LOIC) tool, and started around 5 PM EST on Friday. The site was taken offline 4 and a half minutes after the attack started, but it is yet unclear if the organization took its own site offline or if it was the result of the attack. Either way, Anonymous reached its goal.
According to The Register, the attack was initially organized from the Operation Payback site - tieve.tk - but after this site itself was targeted by an attack (by RIAA's hired guns?), the "center of operations" was moved to anonops.net.
This latest Anonymous assault has been spurred by the recent shutdown of P2P service Limewire, which was the result of a RIAA-initiated law suit.