For a full understanding of the DBCC CHECKIDENT command read Books On Line (BOL) at:
ms-help://MS.SQLCC.v9/MS.SQLSVR.v9.en/tsqlref9/html/2c00ee51-2062-4e47-8b19-d90f524c6427.htm
paying particular attention to what might happed if you use the command improperly ...
If the table is not empty, setting the identity value to a number less than the maximum value in the identity column can result in one of the following conditions:
If a PRIMARY KEY or UNIQUE constraint exists on the identity column, error message 2627 will be generated on later insert operations into the table because the generated identity value will conflict with existing values.
If a PRIMARY KEY or UNIQUE constraint does not exist, later insert operations will result in duplicate identity values.