Concurrent Administrative Operations

Not all administrative tasks are allowed to run concurrently. In the table below, a black circle indicates two operations that cannot run in a database at the same time.

File shrink operations spend most processing time reallocating pages into areas retained after the shrink has completed; it then attempts to change the file size only as the last step. File shrink operations can be started while a backup is running, provided that the backup finishes before the file shrink operation attempts to change the size of the files.

Source : http://technet.microsoft.com/en-us/library/ms189315.aspx