I had applied online and was contacted by an Apple recruiter within a week over email. She gave me details about the position and the name of the team that was interested in my profile. After that, I had a 45-minute telephonic interview with a member of the hiring team. I was asked primarily about my PhD/postdoc research work and was also told about the privacy and confidentiality of Apple about the work done there. The interviewer was very pleasant to talk with and respected my position and what I was looking for.
After the first round of phone interview, Apple scheduled an onsite interview within two weeks in their head office in Cupertino. The pre-interview process was very smooth - they flew me over and arranged a nice hotel for a night stay. My onsite interview lasted for about 5-6 hours, during which I met ten people from the Core OS team, including the hiring manager and the VP. I was asked a variety of questions in the areas of computer networking, wireless communications, and my research work, and also was asked to solve problems on paper and code on the white board. I was able to answer 90% of the questions. Some of the questions required thinking on the spot, and the interviewers were very helpful in giving me hints if I got stuck.
To sum up, Apple has a very smooth overall hiring process and takes care of their potential hires in a nice way. My whole trip was covered by Apple and reimbursed on time. Apple's culture was pleasant and so was the working environment. I was impressed with their Cupertino office environment.
They extended me an offer within two weeks of the onsite interview.