Adding a bit of fuel to the debate surrounding the reliability of cloud services (think Twitter, Google outages), T-Mobile USA has halted sales of its popular messaging-friendly Sidekick device, after a server meltdown essentially erased subscribers’ saved e-mail, photos, address books, calendars and to-do lists. T-Mobile was blunt in a customer statement on Monday, calling the likelihood of recovering the data “extremely low.”

The issue began last week with data service being knocked out and some applications available only on an on-again, off-again basis. T-Mobile is partnered up with Microsoft Corp.’s data management subsidiary Danger to host master copies of subscriber information in the cloud. Customers have been able to back up their files to the remote server, making space locally on their devices as they did so, confident in the process. But then, over the weekend, a massive server failure resulted in a data loss catastrophe.

The statement reads: Regrettably, based on Microsoft/Danger’s latest recovery assessment of their systems, we must now inform you that personal information stored on your device – such as contacts, calendar entries, to-do lists or photos – that is no longer on your Sidekick almost certainly has been lost as a result of a server failure at Microsoft/Danger. That said, our teams continue to work around-the-clock in hopes of discovering some way to recover this information. However, the likelihood of a successful outcome is extremely low.

Some speculate the issue could have been avoided. Even though Danger started having technical troubles last week, it failed to perform a secondary back-up of the data internally, meaning that when the server failure happened over the weekend it was too late to save the information.

Sidekick users still have their locally stored information, but T-Mobile has cautioned customers to avoid running out of battery power, removing the battery, or resetting the device while the servers are still unstable, because this would result in the local information being lost as well. And of course, “all content … that you re-input into your device is not backed up on the network,” T-Mobile’s statement reads.

Unfortunately, there is no easy way for users to back up the information residing on their devices anywhere else, either, aside from e-mailing it to themselves, one piece of information at a time.

No word yet on when the situation will be resolved, but T-Mobile is issuing credits to customers that are the equivalent of the cost of the data plan for one month.

This is the latest in a series of high-profile cloud-based outages, including several for Twitter in the past few months and a series of service disruptions for various Google applications. Those outages however were short (hours, not days) and didn’t result in widespread data loss. The Sidekick fiasco is unfortunately a whole new order of magnitude for cloud failure.