The following shows the accuracy you're talking about...

SELECT 1,CAST('2000-01-01 23:59:59.990' AS DATETIME) UNION ALL

SELECT 2,CAST('2000-01-01 23:59:59.991' AS DATETIME) UNION ALL

SELECT 3,CAST('2000-01-01 23:59:59.992' AS DATETIME) UNION ALL

SELECT 4,CAST('2000-01-01 23:59:59.993' AS DATETIME) UNION ALL

SELECT 5,CAST('2000-01-01 23:59:59.994' AS DATETIME) UNION ALL

SELECT 6,CAST('2000-01-01 23:59:59.995' AS DATETIME) UNION ALL

SELECT 7,CAST('2000-01-01 23:59:59.996' AS DATETIME) UNION ALL

SELECT 8,CAST('2000-01-01 23:59:59.997' AS DATETIME) UNION ALL

SELECT 9,CAST('2000-01-01 23:59:59.998' AS DATETIME) UNION ALL

SELECT 10,CAST('2000-01-01 23:59:59.999' AS DATETIME)

Results:

----------- ------------------------------------------------------

1 2000-01-01 23:59:59.990

2 2000-01-01 23:59:59.990

3 2000-01-01 23:59:59.993

4 2000-01-01 23:59:59.993

5 2000-01-01 23:59:59.993

6 2000-01-01 23:59:59.997

7 2000-01-01 23:59:59.997

8 2000-01-01 23:59:59.997

9 2000-01-01 23:59:59.997

10 2000-01-02 00:00:00.000

(10 row(s) affected)