Now that Windows 8.1 preview is available, people start to think how they could compile W8.1 solutions with TFS. If you use TF Service you need only to enable a new Build Controller in preview, that is enabled to build 8.1 solutions.
Figure 1: Enable the Windows 8.1 Preview Build controller on your TF Service account to build Windows 8.1 applications
It is simple and easy. Now you should only assign the right controller when creating build definition.