The tale of the Sidekick data loss is one fraught with woe, both for T-Mobile and Sidekick users themselves. Initially, knee-jerk user reactions blamed T-Mo for the debacle, but it's starting to look increasingly like the carrier had nothing to do with the crash.
When Microsoft acquired Danger, they also acquired the responsibility to care for their servers which held the Sidekick data. Though T-Mo still acted as the carrier for the Sidekick, data storage was left up to Microsoft. AppleInsider alleges that it was Microsoft's internal failures that led to the crash. In fact, they state, it might even have been an intentional act of sabotage that lead to the data loss in the first place.
Microsoft hasn't come forward to blame any one factor (act of nature, power failure, hardware glitch) for the loss. That, plus the fact that virtually all the data seems to be gone for good, points towards an act of sabotage. AppleInsider states that the long functionality outage, coupled with a near total data loss, was too devastating a failure to be entirely the work of an accident.
There have been rumors of great internal dysfunction and dissidence surrounding the Danger acquisition. Disgruntled employees seeking revenge aren't anything new to the industry. Former Danger employees with an axe to grind and thorough knowledge of their companies database would have been quite capable of orchestrating the outage. The question is, will Microsoft be able to prove it?