It might be wishful thinking, but part of me wonders if Steve Jobs might have actually been able to push back a little and prevented Apple from joining the program at the same time as Microsoft/Google et al. Apple isn't known for being outspoken about privacy, but Jobs is a formidable character to deal with and, well, if anyone had the balls to say 'no', he did.
Apple wasn't added until after Jobs died, years after other major players:
Jobs doesn't have a history of EFF-like activism and has always played ball with the government. I think its a little unhealthy to build him into this superman juggernaut. Yes he was talented and died before his time, but he's no NSA slayer.
To OP's point, if anybody at Apple at all were going to be asked by the government, I have to imagine Jobs would've been left out of the equation. That's one loudmouthed asshole you didn't want keeping secrets for you.
The most surveillance-relevant services Apple provides are probably iMessage, iCloud and FaceTime. All were announced in 2011 and saw widespread adoption in 2012.
I'd noticed that too. Given that Jobs seems to have had the juice to get away with signficant and brazen securities fraud, he may have been in a fairly secure position to say 'no' to PRISM. But for that matter Twitter's CEOs, much less mighty figures, also seem to have been able to say 'no' too. Or maybe the rickety state of Apple's Web services (MobileMe and all that) had something to do with it...
Apple wasn't added until after Jobs died, years after other major players:
http://en.wikipedia.org/wiki/File:Prism_slide_5.jpg