Update #2 (10/15): Microsoft has announced they have been able to recover almost all user data from the Sidekick cloud servers. It had been suggested this data could have been destroyed and thus become unrecoverable, but apparently the potential disaster has been contained to cause a minimal damage, besides the service interruptions Sidekick users have been suffering in the last couple of weeks. Here's an excerpt from Microsoft's official statement:

"We are pleased to report that we have recovered most, if not all, customer data for those Sidekick customers whose data was affected by the recent outage. We plan to begin restoring users personal data as soon as possible, starting with personal contacts, after we have validated the data and our restoration plan. We will then continue to work around the clock to restore data to all affected users, including calendar, notes, tasks, photographs and high scores, as quickly as possible."

Update: T-Mobile recently informed that prospects for recovering 'some' of the lost data is there, hoping that a majority of their customers will be able to recover their personal content. In addition they are offering a free month of data service (only the $20 data plan) and a $100 T-Mobile gift card to those affected.
Original Story is below:
Following up on a host of reported issues that boiled down to massive data loss for many customers, T-Mobile has stepped in to try and stem further issues from arising. Unfortunately for both T-Mobile and potential customers, the short-term solution appears to be a total halt on sales starting immediately. As of last Sunday, T-Mobile listed all Sidekicks as "temporarily out of stock" -- and stores have stopped sales as well.

The standstill will likely continue until a permanent solution is found for the problems leading to data loss. To customers already affected, it's probable that their data is gone forever, and the incident will no doubt harm the trust future customers have in the reliability of T-Mobile, Microsoft and Danger. T-Mobile does claim that only a "minority" of customers were affected by data loss -- but that runs contrary to pulling an entire line of products from store shelves.

That's not to say this is a recall. It isn't, and we can assume the Sidekick will go back up for sale once the issues are sorted out. Given how volatile the smartphone market is right now, I imagine T-Mobile is dedicating a lot of resources to finding a fix.