Take this for the anecdata that it is. I interviewed at Apple, referred by old Microsoft friends that worked there. As I was trying to get a feel for things before the interview, I asked about the software testing. I was told, "don't expect what you're used to at Microsoft". The reference there is from when Microsoft often had more testers on a team than devs (ah, the good ol' days). The summary of what I was told by friends, and the questions I asked during the interview, is that testers at Apple aren't the testers that Microsoft used to have. Microsoft had testers working in MS Research, researching ways to better test software. Apple, from the impressions I got, is doing good to have testers than can write "hello, world". This was from the app side of things, not OS; I don't know if it's any different on the OS side.
But since I don't work there, I have no good inside info. But just from gut feel, I don't think my anecdata is too far off the mark. Based just on the bugs made public, I just don't get the impression that there are testers at Apple whose sole reason for being there is to tear into a piece of software and break it. There was a bug a few weeks ago posted to HN that I commented on. I don't have a link without digging through my comments, but it was something along the lines of "how could a tester not find this in five minutes of exploratory testing?" This bug is similar. It would take more than five minutes, but were this my area to test I'd pick at it once in a while when I had a few minutes. As I pick at it, I wouldn't expect to find anything, but I've got a minute between builds, so instead of randomly clicking Facebook I'll randomly click this dialog. What did the dev forget? What weird state was not accounted for? Some kind of state overflow if I click the button enough times? Shove some Unicode in there, that didn't find anything; meh, maybe I ought to move o...hey, wait a minute. Did that thing just log me in as root?
But my gut says that Apple doesn't employ a lot of testers like that.