At the moment, Apple says "there was no evidence that any data left" its facilities, but that could change as the firm works with law enforcement. There are few details to share right now, but if Apple knows the attack—very narrow in scale, most likely a focused malware infection—is related to Facebook's, then this is a Chinese job.
Curiously (and a little unsettlingly), Apple says it'll release a tool later today allowing customers to check if they're infected with the same malware used in the attack—suggesting it might be some sort of worm that could spread from system to system undetected. [Reuters]
Update: Apple says the infection stemmed from "a vulnerability in the Java plug-in for browsers," meaning some people clicked on a compromised website. Again, not a terribly sophisticated attack.