Recently, Microsoft and T-Mobile had a very embarrassing episode: after a server failure, Sidekick data was lost for a portion of users, and originally Microsoft had said that the data was lost forever.
Shortly after that, Microsoft announced that they were able to recover "most, if not all" of the lost customer data. Now, if you're one of the users who've been affected by this incident, you'll be happy to know that your data will be restored soon.
From Microsoft's official announcement on the matter:
"We continue to make steady progress, and we hope to be able to begin restoring personal contacts for affected users this week, with the remainder of the content (photographs, notes, to-do-lists, marketplace data, and high scores) shortly thereafter."