Postgres doesn't seem to enforce array dimensions

What's up with Postgres and array dimensions?  The table definition
doesn't even store how many dimensions the array should be.

[local]:playpen=> create table arrtest(
playpen(>    id SERIAL PRIMARY KEY,
playpen(>   arrone int[],
playpen(>   arrtwo int[][]
playpen(> );
NOTICE:  CREATE TABLE will create implicit sequence "arrtest_id_seq" for
serial column "arrtest.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index
"arrtest_pkey" for table "arrtest"
CREATE TABLE
Time: 22.688 ms
[local]:playpen=> INSERT into arrtest (arrone) VALUES (null);  --should
work, and does
INSERT 0 1
Time: 3.798 ms
[local]:playpen=> INSERT into arrtest (arrone) VALUES ( ARRAY[1,2]);
INSERT 0 1
Time: 0.668 ms
[local]:playpen=> INSERT into arrtest (arrone) VALUES ( ARRAY[[3,4]]);
--should not work
INSERT 0 1
Time: 0.715 ms
[local]:playpen=> INSERT into arrtest (arrtwo) VALUES ( ARRAY[5,6]);
--should not work
INSERT 0 1
Time: 0.671 ms
[local]:playpen=> INSERT into arrtest (arrtwo) VALUES ( ARRAY[[7,8]]);
INSERT 0 1
Time: 0.704 ms
[local]:playpen=> select * from arrtest;
  id | arrone  | arrtwo
----+---------+---------
   1 |         |
   2 | {1,2}   |
   3 | {{3,4}} |
   4 |         | {5,6}
   5 |         | {{7,8}}
(5 rows)

Time: 1.217 ms
[local]:playpen=>  \d arrtest
                           Table "public.arrtest"
  Column |   Type    |                      Modifiers
--------+-----------+------------------------------------------------------
  id     | integer   | not null default nextval('arrtest_id_seq'::regclass)
  arrone | integer[] |
  arrtwo | integer[] |
Indexes:
     "arrtest_pkey" PRIMARY KEY, btree (id)

[local]:playpen=> select version();
                                                   version

-----------------------------------------------------------------------------------------------------------
  PostgreSQL 8.4.3 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.6
20060404 (Red Hat 3.4.6-9), 32-bit