He wrote about the move to Apple on his personal blog. Krstic was the architect of the Bitfrost security specification used by OLPC for passwords, hard drive encryption, machine authentication, security updates, and prevention of data loss. He will be working on core operating system security in Cupertino.
Though OLPC isn't a large platform that hackers are known for targeting, it's likely the way that Krstic thinks about security that attracted Apple's interest. As ZDNet Asia's sister site ZDNet notes:
Instead of blocking specific viruses, the system (Bitfrost) sequesters every program on the computer in a separate virtual operating system, preventing any program from damaging the computer, stealing files, or spying on the user. Viruses are left isolated and impotent, unable to execute their code.According to Krstic, that "defeats the entire purpose" of creating a virus.