there is a way to do that (normally i dont prefer this method, i would rather
go for other method which i mentioned in my earlier message), 
but it will be  slow when compared to normal sqlloader operation. 
look at following example, so that you will have some idea.if you want information about
records updated, write extra code in function, dont look at 
sqlloader log file for that information.

table structure:

 EMPNO                                                 NOT NULL NUMBER
 ENAME                                                          VARCHAR2(10)
 SAL                                                            NUMBER

EMPNO is primary key

control file :

LOAD DATA
INFILE 'emp1.txt'
APPEND
INTO TABLE emp2
(empno position(01:05) "update_emp(:empno,:ename,:sal)",
 ename position(07:15) ,
 sal position(17:20)) 

create function like below:

create or replace function update_emp(p_empno number,
					p_ename varchar2,
					p_sal number) return number IS
PRAGMA AUTONOMOUS_TRANSACTION;
l_count number;
begin
select count(*) into l_count
from emp2
where empno=p_empno;
if l_count>0 then
 update emp2
 set ename=p_ename,
     sal=p_sal
where empno=p_empno;
end if;
commit;
return p_empno;
end;

my data file (1st time)

12345 suresh    3100 
32456 john king 5100 
12344 robert    6601 
12223 rob       1434 

table's data after first load:

SQL> select * from emp2;

     EMPNO ENAME             SAL
---------- ---------- ----------
     12345 suresh           3100
     32456 john king        5100
     12344 robert           6601
     12223 rob              1434

my data file (second time)

12345 suresh1   4500 
32456 john      7777 
12344 robert    6688 
12223 rob1      1230
12222 kris      3455
23223 kkkk      7266 

table data aftr second load

SQL> select * from emp2;

     EMPNO ENAME             SAL
---------- ---------- ----------
     12345 suresh1          4500
     32456 john             7777
     12344 robert           6688
     12223 rob1             1230
     12222 kris             3455
     23223 kkkk             7266

6 rows selected.

HTH
Suresh Vemulapalli