I was amused by Steve Jobs' Wi-Fi overload issues during his iPhone 4 presentation. While he could ask the audience to "put your laptops on the floor" and turn off their 3G-to-Wi-Fi devices, most cloud providers won't have the luxury of asking customers not to use their services when their cloud platforms get oversaturated.
There have been many recent availability issues with cloud providers, such as Twitter's and Google Calendar's struggles, as well as Google App Engine's datastore taking a dirt nap under demand. Or, as Google puts it in a recent post: "There are a lot of different reasons for the problems [with data store] over the last few weeks, but at the root of all of them is ultimately growing pains. Our service has grown 25 percent every two months for the past six months."
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There are also many cloud outages and availability issues that aren't reported, but have the same negative affects on the cloud users. What we hear in the press is the tip of the iceberg.
I think this increase in outages caused by saturation is just the start. I suspect with the increased use of cloud computing this year and next, clouds falling over due to stress will be more commonplace.
The core issue is the saturation of resources by too many users doing too much on the cloud provider's servers. Putting any architecture and design issues aside for now, it's as simple as that -- it's also a very old problem.