According to this report from CNet, T-Mobile Sidekick users have been unable to access their data, such as calendars, address books, etc. starting more than a week ago. The Sidekick stores most of its users' personal data on servers, rather than on the device itself. Danger, the company that created the Sidekick and operated the servers, was purchased by Microsoft for $500 million last year. Now, according to multiple sources, T-Mobile and Microsoft have notified Sidekick users that the data have most likely been permanently lost; there is a slim but highly unlikely chance that they can be recovered.
This could turn into an unexpected windfall for Google and the Android platform if customers decide to replace their Sidekicks with Android-compatible phones. (In an ironic twist, Andy Rubin, the father of Android, was one of Danger's founders.) Microsoft might try to steer Sidekick customers to Windows Mobile phones, but since Microsoft was responsible for the failure, it's unlikely that they're going to be able to force anyone to go with a Windows phone.
The handwriting has been on the wall for the Sidekick for some time; it's a dead platform. Key members of the Danger team have left Microsoft, and most of the others are now working on the ever-increasingly less secret "Pink" project. However, the platform still had an active base of users and supporters, at least until today.
If T-Mobile and Microsoft don't announce a replacement program soon, expect to see class action lawsuits filed across the country. (Even with a recall, the lawsuits may get filed anyway due to the loss of personal data.) In any event, the Sidekick platform is dead, and the future reliability of any Microsoft-run cloud-based service is in question.
UPDATE 15 October 2009: According to TechCrunch, it now appears that Microsoft has been able to recover most of the lost Sidekick user data, and will begin restoring the information "soon" (the next status update is scheduled for Saturday, so it may be several days or more before users get their data back.)