Wozniak wasn't very clear about what he was worried about, to be fair. He spoke, according to reports (I wasn't at the presentation so can't say for certain) about "terrible things" going to happen over the next five years or so. That's not a very specific definition, so let's have a look at some of the things that are happening already.
We could start with the vanishing YouTube. This isn't strictly a cloud issue as the videos will still be out there, you'll just have to use a bookmark on your phone's browser until YouTube owner Google gets its own app in place. What's actually happened is that Apple's license to make a YouTube app has expired and the company has decided not to renew it.
The same principle applies whenever Facebook, Twitter or anybody else decides to do something they hadn't previously announced and you find your information is affected. If you're on Facebook then you may have been surprised to find that you suddenly had a Facebook email a few weeks ago - firstname.lastname@example.org. Nobody asked for these, they just arrived. Likewise a year or so ago Facebook was coming in for a lot of flack about its privacy policies, which seemed to be changing with the wind.
There was of course a lot of hot air about these policy changes. Some of them gave Facebook the right to reproduce your pictures on its servers; thinking about it, Facebook couldn't possibly show you your own picture if it didn't have the right to keep a copy on its servers, and everyone else who looks at it is technically making a copy on their machine - so licenses had to change to reflect this. Nonetheless people were concerned. The fact was and is that if you're using someone else's system - Facebook or someone else in the cloud - it's their playground and they can change the rules when they want.
It's a little different when you're paying for cloud services somehow, because then you have a service level agreement (SLA - does what it says on the tin). Which will be fine as long as it's working. This is where we get to what I suspect Wozniak was worried about.
If I have an agreement with a service provider - and with Microsoft joining Google in pushing everything to the Web as swiftly as it can we can be certain there will be many such agreements around - then the service provider is going to be obliged to look after my data and business critical information.
Unfortunately circumstances change. Data leaks out - remember a year or so ago Sony leaked a load of user details? Guardian account here, literally millions of customers' information went AWOL. Other stuff changes, like stuff stops working. BlackBerry users found this out to their cost last year - remember this outage. People effectively using technology other than their own were caught, badly. It would be worse if a company on which someone were depending for their mail or other business-critical functions went under completely. Google and others are doing very nicely; by now we'd better hope they continue to do so, because if they go down they'll take a load of us with them as our emails, contacts and in some case office apps vanish completely.
This could be alarmist and Wozniak has been accused of precisely that in some blogs. You can't help but feel he may have a point, though. Increasingly our computers and smartphones are just windows onto other people's services we use. By now there's probably no way round it, the cloud is entrenched into our daily lives. We just need to be aware of it - and make sure any coud providers we use are very trustworthy indeed.