A unified theory of cloud computing isn't as simple as writing software, however. There are a number of social and legal issues that need to be dealt with. Most glaringly is user privacy, says Jimmy Lin, professor of computer science at the University of Maryland. This will become especially important if Google serves ads that correspond to all personal information, as it does in Gmail. Depending on the information used to target the ads, it can make some people uncomfortable. "Different people feel differently about the creepiness factor," says Lin. "My sense is that people are still rather uncomfortable about this." In addition, Google's encryption mechanisms aren't flawless. There have been tales of people logging into Gmail and pulling up someone else's account, he says. "Assurances aren't enough to relieve people's fears. All it would take is one incident to throw this all amok."

Moreover, there are copyright ramifications to cloud computing. One of the advantages to storing data in the cloud is that it can be easily shared with other people, but sharing files such as copyrighted music and movies is generally illegal. And in some cases, says Lin, it's unclear if simply storing information on the cloud violates copyright. For instance, the license restrictions on the data he uses for research makes it illegal to copy it onto computers outside of Maryland. "Here's a case where the legal policy really falls behind," he says. "It'll all have to be rewritten."

There's also the conspicuous issue of constant connectivity: a repository of online data isn't useful if there's no Internet connection to be had, or if the signal is spotty. Google's Schillace says that the Internet is becoming more accessible all the time, but the company is aware of today's technical challenge of providing service to Web applications without interruption. "That's certainly a problem," he says, "but there are a number of ways of addressing it." He says that Google offers downloadable software called Gears that acts as a cache for data when a person is using a Web application offline. When the person regains Internet access, Gears automatically syncs with the online application and saves the work on a server. Schillace's team is working to integrate this sort of feature into Google Docs. Moreover, on Friday, the company confirmed its plans to bid on the 700-megahertz wireless spectrum that will be auctioned off by the Federal Communications Commission in January, potentially providing another avenue allowing people to wirelessly connect.

Microsoft is watching Google's moves closely, and the company has its own approach that it thinks can sidestep some of the cloud-computing challenges. "It's not an online world only," says Brian Hall, the general manager of the Windows Live business group. "It's always going to be a combination of [online and offline], and the solution that wins is going to be the one that does the best job with both." Microsoft, he says, is building software that can let people keep some of their files on hard drives, and maintain that privacy, while still letting them access those files remotely. Hall adds that the amount of storage won't be as important as the user experience. "It's not, in the end, going to be a gigabyte-to-gigabyte competition," he says. "That isn't where the hearts and minds of customers will be won."

Still, there's a sense that Google's version of cloud computing has the appeal of simplicity, in spite of the current challenges. Schillace says that by moving applications and data to the Internet, Google is helping make the computer disappear. "I think every generation of application sort of peels away another layer of the computer," he says. Initially, people interacted with computers using command lines, Schillace explains, then used a graphical interface; now people can do much of their work in a Web browser, which can be on a personal computer or a small handheld device. "It's about letting the computer get out of our way so we can work with other people and share our information."