As Wayner points out, Amazon actually goes as far as to specify that it can terminate its customer agreement at any time, "for any reason or for no reason" (though it promises to give paying customers 60 days' notice before turning off the lights on their apps).
So what, you might say. That's just life in the real world. Businesses will always have suppliers, partnerships, and relationships with other companies. UPS could just as easily stop delivering your packages, or Verizon could cancel your mobile phone contract.
As anyone who manages enterprise applications can tell you however, switching suppliers is never as easy as buying a new phone. This goes double for cloud computing. Currently the market offers only a limited number of vendors, and few or none of your supplier's competitors may be suitable for your purpose.
Migrating your application could also prove to be a challenge. As it stands, each of the current cloud computing services is essentially a proprietary platform, open source tools notwithstanding. Code written for Google's Datastore API won't run at Amazon, for example, and Java applications written for Amazon EC2 won't run at Google. Depending on how you engineered your code, transferring it to a new cloud provider could be difficult or even impossible.
In short, if you're a customer who relies on cloud computing for a mission-critical business process, getting locked out of your application would be akin to coming to work in the morning to find your warehouse roped off with yellow caution tape. "Sorry," says Amazon. "You'll need to find a new warehouse."
Hope you backed up your data.
I don't mean to sound sensational. There's nothing inherently sinister in what Amazon or Google is doing. I can't see how it would make good business sense for either company to hold its customers hostage, either. But if good business planning is about mitigating risk, it's important to recognize the unknown territory that businesses stray into when they enter the nascent world of cloud computing.
There will always be those trailblazers who are first to embrace any new technology, and if the startup community is any indication, the momentum behind cloud computing services is steadily building. The market is yet young, however, and there remains a lot of work to be done before these services are suitable for enterprise customers. Some of that work will be done by clever programmers, but I suspect even more of it rests in the hands of the lawyers.