if you truncate table then NOT NULL constraint will not drop.
Of about not null constriant is not include in constraint it include in TABLE DEFINATION.

SQL> select constraint_name,constraint_type from user_constraints where table_na
me = 'TEE';

CONSTRAINT_NAME                C
------------------------------ -
SYS_C005019                    C
PK_TEE                         P

SQL> host exp scott/tiger file=t.dmp tables=tee

Export: Release 10.1.0.2.0 - Production on Mon Feb 19 08:22:47 2007

Copyright (c) 1982, 2004, Oracle.  All rights reserved.


Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Produc
tion
With the Partitioning, OLAP and Data Mining options
Export done in WE8MSWIN1252 character set and AL16UTF16 NCHAR character set

About to export specified tables via Conventional Path ...
. . exporting table                            TEE          2 rows exported
Export terminated successfully without warnings.

SQL> truncate table tee;

Table truncated.

SQL> alter table tee drop constraint SYS_C005019;

Table altered.

SQL> alter table tee drop constraint PK_TEE;

Table altered.

SQL> host imp scott/tiger file=t.dmp full=y IGNORE=Y

Import: Release 10.1.0.2.0 - Production on Mon Feb 19 08:24:11 2007

Copyright (c) 1982, 2004, Oracle.  All rights reserved.


Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Produc
tion
With the Partitioning, OLAP and Data Mining options

Export file created by EXPORT:V10.01.00 via conventional path
import done in WE8MSWIN1252 character set and AL16UTF16 NCHAR character set
. importing SCOTT's objects into SCOTT
. . importing table                          "TEE"          2 rows imported
Import terminated successfully without warnings.

SQL> select constraint_name,constraint_type from user_constraints where table_na
me = 'TEE';

CONSTRAINT_NAME                C
------------------------------ -
PK_TEE                         P

SQL>

--------------------------------------------------------
Now we drop table and import with table defination.
SQL> drop table tee purge;

Table dropped.

SQL> host imp scott/tiger file=t.dmp full=y

Import: Release 10.1.0.2.0 - Production on Mon Feb 19 08:25:09 2007

Copyright (c) 1982, 2004, Oracle.  All rights reserved.


Connected to: Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Produc
tion
With the Partitioning, OLAP and Data Mining options

Export file created by EXPORT:V10.01.00 via conventional path
import done in WE8MSWIN1252 character set and AL16UTF16 NCHAR character set
. importing SCOTT's objects into SCOTT
. . importing table                          "TEE"          2 rows imported
Import terminated successfully without warnings.

SQL> select constraint_name,constraint_type from user_constraints where table_na
me = 'TEE';

CONSTRAINT_NAME                C
------------------------------ -
SYS_C005022                    C
PK_TEE                         P

SQL>