Conditional left join

I have the following table structure on Postgres 8.4 :

STRUCTURE: tblunit
unit_id [integer]
unit_location [character varying]

DATA:
1,'location1'
2,'location2'
3,'location3'

STRUCTURE: tbloperator
operator_id [integer]
operator_name [character varying]

DATA:
1,'operator1'
2,'operator2'
3,'operator3'
4,'operator4'
5,'operator5'
6,'operator6'

STRUCTURE: tbloperatorschedule
operator_schedule_id [bigint]
operator_id [integer] {Foreign key tbloperator->operator_id}
schedule_start_time [timestamp without time zone]
schedule_end_time [timestamp without time zone]
unit_id [bigint] {Foreign key tblunit->unit_id}

DATA:
1,1,'2011-12-01 01:00:00','2011-12-01 02:00:00',1
2,5,'2011-12-01 02:30:00','2011-12-01 04:50:00',1
3,2,'2011-12-01 04:55:00','2011-12-01 10:20:00',1
4,1,'2011-12-01 03:00:00','2011-12-01 05:00:00',2
5,3,'2011-12-01 05:30:00','2011-12-01 09:50:00',2
6,4,'2011-12-01 09:55:00','2011-12-01 13:20:00',2
7,6,'2011-12-01 14:00:00','2011-12-01 18:00:00',2
8,5,'2011-12-01 06:30:00','2011-12-01 14:50:00',3
9,2,'2011-12-01 14:55:00','2011-12-01 20:20:00',3

STRUCTURE: tbldata
data_id [bigint]
event_time [timestamp without time zone]
data_text [character varying]
unit_id [bigint] {Foreign key tblunit->unit_id}

DATA:
1,'2011-12-01 02:30:00','abc',1
2,'2011-12-01 06:28:00','abc',2
3,'2011-12-01 11:10:00','abc',3
4,'2011-12-01 21:30:00','abc',3

I am trying (through conditional left join?) to fetch all records of
tbldata and the operator name from tbloperators who was operating the unit
at event time. If no operator was present, it should return null.

Resulting set:
1,'2011-12-01 02:30:00','abc',1,operator5
2,'2011-12-01 06:28:00','abc',2,operator3
3,'2011-12-01 11:10:00','abc',3,operator5
4,'2011-12-01 21:30:00','abc',3,NULL

The sql query to create the tables and sample data is attached or can be
seen at http://pastebin.com/Fy2t3H9S . Is it possible to get a condition
within a left join or any other way to fetch the desired data.

Amitabh