Return the latest transactions for all parts

    CREATE TABLE #Parts
        (
        [partLog_ID] INT IDENTITY(1,1),
        [Part Number] CHAR(5),
        [Quantity on Hand] INT,
        [Updated Date] DATETIME
        )


    INSERT INTO #parts SELECT 'SC198', 30, '1/2/2002'
    INSERT INTO #parts SELECT 'SC198', 50, '1/5/2003'
    INSERT INTO #parts SELECT 'SC198', 60, '2/3/2007'
    INSERT INTO #parts SELECT 'SC198', 70, '2/6/2008'
    INSERT INTO #parts SELECT 'SC198', 100, '3/6/2010'
    INSERT INTO #parts SELECT 'SC198', 200, '5/2/2010'
    INSERT INTO #parts SELECT 'SC216', 10, '1/2/2003'
    INSERT INTO #parts SELECT 'SC216', 20, '2/3/2007'
    INSERT INTO #parts SELECT 'SC216', 50, '1/1/2010'
    INSERT INTO #parts SELECT 'SC216', 80, '3/2/2010'
    INSERT INTO #parts SELECT 'SC216', 60, '4/5/2010'

    SELECT
      #parts.*
    FROM
      #parts
      INNER JOIN (SELECT
                    [part Number], MAX([Updated Date]) AS [final date]
                  FROM #parts
                  WHERE   [Updated Date] BETWEEN '1 jan 2002' AND '1 jul 2010'
                  GROUP BY
                    [part Number]
                 ) f
        ON f.[part Number]=#parts.[Part Number]
           AND [Updated Date]=[final date]

    /*
    partLog_ID  Part Number Quantity on Hand Updated Date
    ----------- ----------- ---------------- -----------------------
    11          SC216       60               2010-04-05 00:00:00.000
    6           SC198       200              2010-05-02 00:00:00.000
    */*/

This will give you what you want for dates before 2010/05/05 - obviously just change the date as you want. You haven't specified what SQL Server version you're on - this is a 2005+ query as it makes use of the windowed function 'ROW_NUMBER'

DECLARE @EndDate datetime
SET @EndDate = '20100505'

SELECT PartNumber, QtyOnHand, UpdatedDate
  FROM (SELECT PartNumber,  
          QtyOnHand,  
          UpdatedDate,  
          ROW_NUMBER() OVER (PARTITION BY PartNumber ORDER BY UpdatedDate DESC)  
              as __RN
         FROM MyTable
         WHERE UpdatedDate < @EndDate
       ) iDat
 WHERE __RN = 1

How about

SELECT TOP(1) ColumnWhatever
FROM MyTable
WHERE UpdatedDate < '3/1/2010'
ORDER BY UpdateDate DESC