T-Mobile, Microsoft and the Redmond company's Danger subsidiary have been hit with the first lawsuits over the data meltdown that has left many Sidekick users unable to access their address books, calendars and other personal information. Ina Fried of CNet News.com broke the news this afternoon, citing two cases filed on behalf of consumers this week U.S. District Court in Northern California.

Here's a copy of one (PDF, 21 pages), filed on behalf of Maureen Thompson, a Georgia resident who experienced a "complete and catastrophic loss of all data on her daughter's Sidekick." The suit claims that the architecture of Danger's Sidekick data system contributed significantly to the problem. Here's an excerpt.

Sidekicks retrieve information as necessary from the Microsoft/Danger servers and cache temporary copies on the Sidekicks. The copies of user data stored on Sidekicks do not serve as permanent copies of the data stored on Microsoft/Danger's servers. Instead, they are mere temporary copies but mirror the master copies on Microsoft/Danger's servers. In the event the master copies are destroyed, as happened in this case, the temporary cached copies will also be destroyed, particularly and necessarily on device re-boot.

Further complicating the data loss is the fact that Sidekicks, unlike iPhones, BlackBerrys and other smartphones, are not designed to sync locally with a user's personal computer without additional software and hardware. This means that most users were not able to backup their data locally, but were encouraged and required to rely on Microsoft/Danger.

The suit alleges negligence, among other things, and seeks class-action status.

Microsoft has said the problem was caused by a Danger server failure.

T-Mobile on Monday expressed optimism about the possibility of restoring the data. The company is promising a $100 "customer appreciation card" to anyone whose data can't be recovered, in addition to the month's worth of free data service already extended to Sidekick users.

Previously: Microsoft says it had Sidekick data backup, but it was hit, too