how to "group" several records with same timestamp into one line?

Hi everyone,
My data with same timestamp "2008-11-12 12:12:12" in postgresql are as
follows

rowid data unit channel create_on
------------------------------------------------------------------------------
1 1.5 MPa channel1 2008-11-12 12:12:12
2 2.5 M3 channel2 2008-11-12 12:12:12
3 3.5 M3 channel3 2008-11-12 12:12:12
4 4.5 t channel4 2008-11-12 12:12:12
------------------------------------------------------------------------------

I would like to "group" them into one line with SQL like

1.5 MPa 2.5 M3 3.5 M3 4.5 t 2008-11-12 12:12:12

The table is defined as

CREATE TABLE record_data
(
rowid serial NOT NULL,
data double precision,
unit character(10),
channel character(20),
create_on timestamp
)

Is that possible? Thanks for your help in advance.

Ouyang