The logo for MobileMe, Apple’s upgrade to its .Mac suite of Web applications, comprises the icons of four of the service’s major features–e-mail, calendar, address book, and photo gallery–floating in a cottony white cumulus. The image announces an elegant, user-friendly take on cloud computing, syncing users’ data wirelessly between Macs and PCs, the Web, iPhones, and iPod touches.

MobileMe has garnered positive reviews for its features and its intuitive user interface. But accessing Apple’s cloud has been a stormy experience for some users. The service’s debut on July 10 was marred by delays and missing features. Apple apologized for the launch troubles, but since then some users have altogether lost access to their e-mail.

The company also backed off from using the term “push”–which implies near-instantaneous synchronization between devices–in describing the service. While MobileMe does push changes made on the Web or an iPhone or iPod touch, changes made from a Mac or PC can take up to 15 minutes to propagate.

Apple was contacted for this article but was unavailable for comment.

Cloud computing has been touted as a potential tool for everything from improving business infrastructure to helping consumers keep tabs on their contacts. Storing data in the “cloud” of the Internet rather than locally allows users to access that information anywhere and at any time.

Some cloud-computing applications–like Google’s Gmail, Google Calendar, and the Google Docs document-sharing and -editing service–live entirely in the cloud: users’ data is stored remotely and accessed via a Web browser. Other applications–like the contact-syncing service Plaxo–use the cloud to back up data and keep it up to date across multiple computers and mobile devices.

MobileMe combines both approaches, syncing data between computers while providing access to a user’s e-mail, contacts, calendar, and photos via the Web. But the service’s troubles illustrate an obstacle to the mass acceptance of cloud computing: the average user has a low tolerance for downtime.

“Availability is essential in cloud computing,” says Thomas Vander Wal, founder of the IT consultancy InfoCloud Solutions. “If constant access to information and objects is a requirement, then cloud computing may not be a viable option without alternate solutions.” The problem is not limited to Apple. Vander Wal notes that a July 20 outage on Amazon’s Simple Storage Service (S3) affected a host of Web-based applications that use it to store data online.

One product affected by the outage was Dropbox, a file-synchronization and -backup service for Macs and PCs. “Syncing and Web access to files was offline during the S3 outage,” says Dropbox founder Drew Houston. “But Dropbox stores all files locally, so users could still access and change their files, and queued changes synced immediately after S3 returned.”

Dropbox’s ability to respond to the outage highlights a difference between services that exist only in the cloud and those that use the cloud to keep devices current. “Sync-based solutions are somewhat more tolerant of minor outages, whereas online-only applications are completely sensitive to downtime,” says John McCrea, vice president of marketing for Plaxo.

This point is not lost on Google, which is developing a product–Google Gears–that brings the resiliency of synchronization to its suite of Web-based applications. “Where Google is moving with Gears, which provides the ability to work locally and sync and update the cloud when there is connection, is a viable way forward,” says InfoCloud’s Vander Wal.

Regardless of how much redundancy developers can build into their applications, the question remains: are users ready to trust their data to the cloud? Vander Wal is skeptical. “A lot of the conceptual models just aren’t there in people’s heads,” he says.

The trick, says Dropbox’s Houston, is to make the transition as familiar and seamless as possible. “The cloud will make a lot of things easier, but it’s less useful if you have to change your behavior or can’t use the apps you need,” he says. What people need, Houston says, is a solution that “just works.”