T-Mobile USA, in cooperation with Danger, a subsidiary of Microsoft, made some apologies over the weekend.

Danger, the data services provider for the Sidekick line of phones, lost a significant amount of user information and lacks any kind of redundant back-up for the recovery of that information, according to a letter to customers.

The companies are pretty sure all of the Sidekick users’ data was lost, which includes contacts, calendar entries, to-do lists and photos. The cause of the loss appears to have been “a result of a server failure at Microsoft/Danger.” Microsoft acquired Danger for $500 million back in February of 2008.

While T-Mobile said that it continues to work on recovering the information, the company admits that “the likelihood of a successful outcome is extremely low.” The company also offered tips on its Web site that it hopes will help customers deal with the crisis. However, the Sidekick stores almost all of its information in the cloud, which means there is little hope for users to do anything on their end.

The notice to customers gave subtle indications that users may be compensated for their trouble. “We also are considering additional measures for those of you who have lost your content to help reinforce how valuable you are as a T-Mobile customer,” stated the release.

The case represents a worst-case scenario as companies begin exploring cloud-based services and storage.

Update: According to a T-Mobile spokeswoman, all Sidekick users impacted by the loss of data will receive credit for one month of Sidekick data service. She said T-Mobile is also considering additional measures to help reinforce how much the carrier values their business.