After connecting the device to an Internet connection and USB storage medium, simply go to the Pogoplug web-site and you can securely access your data. While this is nice for making it an OS-neutral environment in which you can access your data (they also have an open API), there is no program at this time you can run on a computer to mount the remote storage device. If Pogoplug's servers are down, the user is unable to access their data over the Internet since their little hardware device would not work.
Rather than coming out with an open-source software stack for the Pogoplug at this time, the company behind it (CloudEngines), has promised the source-code but only if they fail. As they publicly announced this morning, if they go out of business they will go open-source. They have created a source-code escrow account to house the code in the mean time, but in the event they go under, it will be published to the beloved SourceForge. We are told by the company that their source-code escrow account will be continually updated against their latest in-house code.