Microsoft issued an official statement regarding the T-Mobile Sidekick Data Loss disaster. The statement is signed by VP Premium Mobile Experience Roz Ho and says that Microsoft recovered most, if not all, customer data for those Sidekick customers whose data was affected by the recent outage.

Microsoft plans to begin restoring personal data as soon as possible, starting with personal contacts, after the company has validated the data and restoration plan.
Microsoft has determined that the outage was caused by a system failure that created data loss in the core database and the back-up. Microsoft has rebuilt the system component by component, recovering data along the way. This careful process has taken a significant amount of time, but was necessary to preserve the integrity of the data.

Microsoft assumes now that only a minority of Sidekick users will be affected by the data loss. If your data is lost you should continue to check for updates in the T-Mobile Sidekick forums.

At least Microsoft took the issue seriously, but it took days before they actually had a solution in place. The damage is already done, but I guess most Sidekick owners will stay with the device going forward, despite tempting offers from Pronto.