The resilience of the cloud as a data storage medium was called into question this week, when T-Mobile USA’s Sidekick users were told their personal information and content had been irretrievably lost.

T-Mobile and the Sidekick data services provider, Danger, a subsidiary of Microsoft, issued a grovelling apology, after a glitch wiped out user data stored in the cloud.

The Sidekick is a niche messaging-focused gadget with a flip out QWERTY keyboard. It is pitched at the youth market and is promoted by the likes of Paris Hilton. Historically however, the device is interesting in that Andy Rubin, founder of Sidekick and Hiptop developer Danger, went on to form a highly secretive mobile software firm known as Android that was snapped up by Google in 2005.

“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,” the service providers said.

The Danger service backs up all content stored on the device – contacts, photos, messages etc. to a cloud computing system run by the providers. Cloud computing is all the rage, and is a strategy being pursued by almost all players in the mobile space including Google, Nokia, Palm, Apple, as well as the multitude of operators that offer back up services to their customers.

Yet the outage suffered by T-Mobile users suggests that the technology is not as infallible as some proponents believe. Analysts have said that cloud computing will evolve to gradually fill more and more computing service needs, but there are still reservations at present. On Monday, Gartner analyst Thomas Bittman warned that cloud computing services still suffer from “service level requirements that can’t always be met, glaring security holes exist, [and] regulatory compliance requirements haven’t caught up with technological capability.”

T-Mobile has withdrawn the Sidekick from sale, and advises users to not reset their device by removing the battery or letting their battery drain completely, as any personal content that currently resides on the handset will be lost.

The carrier said it will send affected customers a $100 voucher in addition to a month of free data service.