T-Mobile USA and Microsoft issued a statement notifying users that "existing T-Mobile's Sidekick devices users will still be able to make phone calls and send text messages, but after May 31 they'll lose access to the cloud-based data service for their contacts, photos, calendar items and notes."
Here's the full statement from the companies:
After May 31, 2011, the Danger Service (a subsidiary of Microsoft) used by T-Mobile Sidekick customers for data services will no longer be available on Sidekick devices.
T-Mobile will provide offers for our Sidekick customers before May 31, 2011, to help make an easy transition from their existing Sidekick device to a new device. We will have more information to share about these offers with our customers in the weeks ahead.
To ensure the best possible transition for our loyal Sidekick customers, an enhanced Web tool is available on myT-Mobile.com to easily export their personal data, including contacts, photos, calendar, notes, to-do lists, and bookmarks, from the Danger service to a new device, computer, or a designated e-mail account. An application is also available in the Sidekick Catalog to make it easy to export personal data to the Sidekick's memory card. Many T-Mobile stores can transfer data from that card to a new T-Mobile device if the customer brings in the memory card and Sidekick.