Utilizando a View no SQLServer 2000 e 2005

SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS

Select puro no SQLServer 2005

SELECT OBJECT_NAME(OBJECT_ID) AS NameofConstraint,
            SCHEMA_NAME(schema_id) AS SchemaName,
            OBJECT_NAME(parent_object_id) AS TableName,
            type_desc AS ConstraintType
    FROM sys.objects
    WHERE type_desc LIKE '%CONSTRAINT'