The online service backs up contacts, photos, calendar appointments and other personal data stored on the mobile phones of Sidekick T Mobile customers. The data is stored on Microsoft's servers, not in the handheld device itself.
Microsoft Corporate Vice President Roz Ho says that all data will be restored, beginning with personal contacts. She believes that only a minority of Sidekick users are still affected.
You can read more at http://news.bbc.co.uk/2/hi/technology/8309218.stm.