They just get repurposed. At least if they were already modern enough to become the basis for a new operation.
Back in 2009, JetBlue Airways made the financially sound choice for them. Continued growth and the need to support the widely geographically dispersed employees of the airline meant that they needed a much better networking and datacenter infrastructure. Rather than building it all themselves, despite the fact that they already had two datacenter locations, they decided to contract with Verizon to provide their networking and datacenter operations. This choice meant that their datacenter in Garden City, New York, which was their flight operations center, would be going dark.
Given the nature of JetBlue's operations there, this datacenter facility had to be fully redundant on all levels, so there had been a considerable investment in the infrastructure of the facility to allow for 24/7 operations with high availability. But what do you do with this kind of facility when you no longer need it (which is not the same question as deciding if the facility has reached its end of life).
Enter Webair, a NY -based hosting provider that was looking to expand their operations. Today, less than 16 months after JetBlue made the decision to move out, Webair announced that they have opened up the facility as their new flagship datacenter and executive headquarters, basing their Network Operations Center in the new (to them) facility which they have dubbed "NY1". Webair has posted what they call a virtual tour of their new facility (actually just a photo essay) that can be found here.
The managed hosting and datacenter business continues to grow and this early 2011 announcement bodes well for the health of the managed hosting business as well as the future need for datacenter services.