The demonstrations are usually proofs of concept that never seem to actually get exploited in the wild. It may seem that the experts are like the hackers who cried "wolf" -- but if you remember how that story ended, it wasn't a happy outcome for the sheep or the boy.
This year, security researcher Dai Zovi showed a way for hackers to take control of Safari and steal encrypted data. But more worrying is his prediction that as the Mac market share grows and hackers start targeting them, Macs will prove to be more vulnerable than we've come to expect. Reuters quoted Dai Zovi as saying, "There is no magic fairy dust protecting Macs."
His prediction is seconded by Charlie Miller, who wrote The Mac Hacker's Handbook. He said, "[Apple's security efforts] are advancing. Our concern is that they are just not advancing as fast as they are gaining market share."
Black Hat also saw the demo of a method of taking control of an iPhone by sending malicious SMS messages. The vulnerability was discovered by the same Charlie Miller, who said "SMS is an incredible attack vector for mobile phones. All I need is your phone number. I don't need you to click a link or anything."
And last week, two researchers posted a video on YouTube demonstrating how easy it can be to break the encryption and retrieve the passcode an on iPhone 3GS -- the same encryption that Apple describes as "highly secure."
The takeaway here isn't that Macs and iPhones are particularly vulnerable. It's that they aren't as invulnerable as we Mac users, who've never had to confront serious attacks on our machines, have come to expect. Don't assume your sheep, I mean your Macs, are safe just because the previous warnings didn't pan out.