confusing. it is not changed in GLOBAL, but changed in SESSION. I also use MySQL 5.5, CentOS, 64Bit. anyway if you want set this value permanently, add `set bulk_insert_buffer_size=256M' and restart.
mysql> show variables like '%bulk%';
+-------------------------+-----------+
| Variable_name | Value |
+-------------------------+-----------+
| bulk_insert_buffer_size | 268435456 | <= initial value
+-------------------------+-----------+
1 row in set (0.00 sec)
mysql> set global bulk_insert_buffer_size = 1024 * 1024 * 1;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like '%bulk%';
+-------------------------+-----------+
| Variable_name | Value |
+-------------------------+-----------+
| bulk_insert_buffer_size | 268435456 | <= not changed with GLOBAL
+-------------------------+-----------+
1 row in set (0.00 sec)
mysql> set session bulk_insert_buffer_size = 1024 * 1024 * 1;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like '%bulk%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| bulk_insert_buffer_size | 1048576 | <= changed with SESSION
+-------------------------+---------+
1 row in set (0.00 sec)
MySQL manual says bulk_insert_buffer_size
can be modified while MySQL running (Dynamic Variable
= yes). SESSION and GLOBAL
ly changable.

UPDATED
IF bulk_insert_buffer_size
is changed via GLOBAL, It seems that not affect SESSION variable (still confusing). To see, use "SHOW GLOBAL VARIABLE", "SHOW VARIABLES" implies "SHOW SESSION VARIABLES"
mysql> show session variables like '%bulk%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| bulk_insert_buffer_size | 2097152 |
+-------------------------+---------+
1 row in set (0.00 sec)
mysql> show global variables like '%bulk%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| bulk_insert_buffer_size | 1048576 |
+-------------------------+---------+
1 row in set (0.00 sec)
mysql> show variables like '%bulk%';
+-------------------------+---------+
| Variable_name | Value |
+-------------------------+---------+
| bulk_insert_buffer_size | 2097152 |
+-------------------------+---------+
1 row in set (0.00 sec)