scott@9i > alter user scott account lock;

User altered.

scott@9i > select username,account_status from dba_users where username='SCOTT';

USERNAME                       ACCOUNT_STATUS
------------------------------ --------------------------------
SCOTT                          LOCKED

oracle@mutation#exp system/manager owner=scott

Export: Release 9.2.0.4.0 - Production on Wed Nov 23 11:43:40 2005

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.


Connected to: Oracle9i Enterprise Edition Release 9.2.0.4.0 - 64bit Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
Export done in US7ASCII character set and AL16UTF16 NCHAR character set
server uses WE8ISO8859P1 character set (possible charset conversion)

About to export specified users ...
. exporting pre-schema procedural objects and actions
. exporting foreign function library names for user SCOTT
. exporting PUBLIC type synonyms
. exporting private type synonyms
. exporting object type definitions for user SCOTT
About to export SCOTT's objects ...
. exporting database links
. exporting sequence numbers
. exporting cluster definitions
. about to export SCOTT's tables via Conventional Path ...
. . exporting table                           DEPT          4 rows exported
. . exporting table                            EMP         14 rows exported
. . exporting table                        MYTABLE          3 rows exported
. . exporting table                     PLAN_TABLE          0 rows exported
. exporting synonyms
. exporting views
. exporting stored procedures
. exporting operators
. exporting referential integrity constraints
. exporting triggers
. exporting indextypes
. exporting bitmap, functional and extensible indexes
. exporting posttables actions
. exporting materialized views
. exporting snapshot logs
. exporting job queues
. exporting refresh groups and children
. exporting dimensions
. exporting post-schema procedural objects and actions
. exporting statistics
Export terminated successfully without warnings.

--
-- Let us see how far this is a fact.
--
scott@9i > create user XDB identified by test;

User created.

scott@9i > !exp scott/tiger owner=XDB

Export: Release 9.2.0.4.0 - Production on Thu Dec 1 07:28:44 2005

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.


Connected to: Oracle9i Enterprise Edition Release 9.2.0.4.0 - 64bit Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
Export done in US7ASCII character set and AL16UTF16 NCHAR character set
server uses WE8ISO8859P1 character set (possible charset conversion)

About to export specified users ...
EXP-00010: XDB is not a valid username
Export terminated successfully with warnings.

-- 
-- Why is this behaviour?
-- a quick look into catexp.sql , explains it.
-- You can see several users are specifically excluded from being exported.

   680  CREATE OR REPLACE VIEW exu8usr (
   681                  name, userid, passwd, defrole, datats, tempts, profile#,
   682                  profname, astatus, ext_username) AS
   683          SELECT  u.name, u.user#, DECODE(u.password, 'N', '', u.password),
   684                  DECODE(u.defrole, 0, 'N', 1, 'A', 2, 'L', 3, 'E', 'X'),
   685                  ts1.name, ts2.name, u.resource$, p.name, u.astatus,
   686                  u.ext_username
   687          FROM    sys.user$ u, sys.ts$ ts1, sys.ts$ ts2, sys.profname$ p
   688          WHERE   u.datats# = ts1.ts# AND
   689                  u.tempts# = ts2.ts# AND
   690                  u.type# = 1 AND
   691                  u.resource$ = p.profile# AND
   692                  u.name NOT IN ('ORDSYS', 'MDSYS', 'CTXSYS', 'ORDPLUGINS',
   693                                 'LBACSYS', 'XDB')
   694  /