WITH t AS (
  SELECT TO_DATE('1/1/2003','mm/dd/yyyy') start_date,TO_DATE('1/4/2005','mm/dd/yyyy') end_date FROM dual 
  UNION ALL           
  SELECT TO_DATE('1/1/2004','mm/dd/yyyy') start_date,TO_DATE('5/3/2006','mm/dd/yyyy') end_date FROM dual 
  UNION ALL
  SELECT TO_DATE('1/1/2007','mm/dd/yyyy') start_date,TO_DATE('1/4/2008','mm/dd/yyyy') end_date FROM dual 
  UNION ALL
  SELECT TO_DATE('1/1/2008','mm/dd/yyyy') start_date,TO_DATE('6/3/2009','mm/dd/yyyy') end_date FROM dual
          )
SELECT  min(start_date) start_date,
        max(end_date)   end_Date
  FROM  (
         SELECT  start_date,
                 end_date,
                 sum(start_of_group) over(order by start_date,end_date) grp
           FROM  (
                  select  start_date,
                          end_date, 
                          case
                            when start_date 
 > 1 + max(end_date) over(order by start_date,end_date rows between unbounded preceding and 1 preceding) then 1
                          end start_of_group
                    FROM  t
                 )
        )              
  GROUP BY grp
  ORDER BY 1
/

START_DAT END_DATE
--------- ---------
01-JAN-03 03-MAY-06
01-JAN-07 03-JUN-09

SQL> 

WITH t AS (
  SELECT TO_DATE('1/1/2003','mm/dd/yyyy') start_date,TO_DATE('1/4/2005','mm/dd/yyyy') end_date FROM dual UNION ALL
  SELECT TO_DATE('1/1/2004','mm/dd/yyyy') start_date,TO_DATE('5/3/2006','mm/dd/yyyy') end_date FROM dual UNION ALL
  SELECT TO_DATE('1/1/2007','mm/dd/yyyy') start_date,TO_DATE('1/4/2008','mm/dd/yyyy') end_date FROM dual UNION ALL
  SELECT TO_DATE('1/1/2008','mm/dd/yyyy') start_date,TO_DATE('6/3/2009','mm/dd/yyyy') end_date FROM dual
          )
SELECT  start_date,
        end_date
  FROM  t
  MODEL
  RETURN UPDATED ROWS
  DIMENSION BY(
               ROW_NUMBER() OVER(ORDER BY start_date) rn
              )
  MEASURES(
           start_date,
           end_date,
           1 i
          )
  RULES ITERATE(1e9) UNTIL(start_date[iteration_number + 2] IS NULL)
    (
     end_date[i[1]] = CASE
                        WHEN start_date[iteration_number + 2] <= end_date[i[1]] + 1
                          THEN GREATEST(end_date[i[1]],end_date[iteration_number + 2])
                          ELSE end_date[i[1]]
                      END,
     i[1]           = CASE
                        WHEN start_date[iteration_number + 2] > end_date[i[1]] + 1
                          THEN iteration_number + 2
                          ELSE i[1]
                      END
    )
  ORDER BY end_date
/

START_DAT END_DATE
--------- ---------
01-JAN-03 03-MAY-06
01-JAN-07 03-JUN-09

SQL>