In an interview with New York Times, Sony CEO Jack Tretton has said that 90% of customers have returned to PSN. "We have over 90 percent of our customers back right now - more than 70 million people. We expect you'll see an influx of new people as new games come out this year," he said.
There are still no news about who actually performed the PSN hack, though. "We still have no insights into who attacked us," Tretton said when asked about it.