Vendor lock-in is an issue with any data storage system - in the cloud or hosted in-house. We need to look into and investigate the tools that the vendor provides to extract the data out of the system.
From what I seen (and experimented with), Google provides excellent set of APIs to access the data stored in Google's Cloud. And Google is always working on to improve the APIs. Google usually first adds functions to the API, and then introduces them in the UI. Compare this to other software vendors, who usually introduce the new functions in the UI and then at a later time provide API access to those functions - if it all.
I currently use both Google Docs and Windows Live Workspace to store my personal / school related stuff. I use both of these because they both have their benefits. Windows Live Workspace provides complete integration with Office 2007, whereas Google Docs provide editing capabilities in a Web browser. Recently I have been thinking of writing an application that will synchronize the content of both of these repositories. Google provides APIs that make this task easy from Google's side, but there are no Windows Live Workspace APIs, so I have to devise a workaround to get documents into the Windows Live Workspace.
No amount of precautions can avoid problems that we do not yet foresee. We need to find solutions to the problems, not just avoid them. An ounce of prevention equals a pound of cure, but that's only if we know what to "prevent". We should be looking into ways to reduce the security and privacy risks associated with Cloud Computing and improve data-portability- efficiently and cheaply.
With problems that we are not aware of yet, the ability to put right - not the sheer good luck of avoiding indefinitely - is our only hope, not just of solving problems, but of making progress. - Physicist David Deutsch
Oh btw, also check out Data Liberation Front