“Consistent with our security response practices”, the company chose not to make a detailed statement during the initial information gathering process. According to the few information available, a small number of computers, including several machine in the Mac business unit, were infected by malicious software using techniques similar to those documented by other organizations.
This suggests that the company was probably the victim of the exploit injected through the compromising of the iPhoneDevSDK Forum. Apparently there is no evidence of customer data being affected while the investigation is ongoing.
Only the last example of an endless trail of high-profile security breaches.