plpython trigger not working

I'm trying to implement a plpython trigger so that I can use a third part
application, using Postgresql 7.3.3 on RH8.0, with Python 2.2.1.
It's supposed to change the 'modified' field to true whenever a record is
modified or added in this table:

pilot-db=# \d timelog_data ;
               Table "public.timelog_data"
     Column      |           Type            | Modifiers
-----------------+---------------------------+-----------
 task            | character varying(255)    |
 time            | time(0) without time zone |
 date            | date                      |
 palm_record_id  | integer                   |
 record_modified | boolean                   |

Here's the trigger:

pilot-db=# \df+ pilotdb*
                                                                  List of
functions
 Result data type | Schema |      Name       | Argument data types |  Owner
| Language |                 Source code                  | Description

------------------+--------+-----------------+---------------------+----------+----------+----------------------------------------------+-------------
 "trigger"        | public | pilotdb_trigger |                     | postgres
| plpython | TD["new"]["modified"] = "t"; return "MODIFY" |

Here's the before and after:
pilot-db=# select * from timelog_data;
     task     |   time   |    date    | palm_record_id | record_modified
--------------+----------+------------+----------------+-----------------
 Network      | 11:14:00 | 2003-06-13 |        8933378 | f
 Tech Support | 14:16:00 | 2003-06-13 |        8933379 | f
 End Tasks    | 14:16:00 | 2003-06-13 |        8933380 | f
 End Tasks    | 11:44:00 | 2003-06-15 |        8933377 | f
(4 rows)

pilot-db=# update timelog_data SET date = '2003-06-16' where palm_record_id =
'8933377';
UPDATE 1
pilot-db=# select * from timelog_data;
     task     |   time   |    date    | palm_record_id | record_modified
--------------+----------+------------+----------------+-----------------
 Network      | 11:14:00 | 2003-06-13 |        8933378 | f
 Tech Support | 14:16:00 | 2003-06-13 |        8933379 | f
 End Tasks    | 14:16:00 | 2003-06-13 |        8933380 | f
 End Tasks    | 11:44:00 | 2003-06-16 |        8933377 | f
(4 rows)

Can anyone please help me understand what I'm missing here?

Cordially,
Scott