Update table with random values from another table

I have a test system for which I need to replace actual user's data (in
'users') with anonymised data from another table ('testnames') on
postgres 8.3.

The tricky aspect is that each row from testnames must be randomised to
avoid each row in users being updated with the same value.

I've been trying to avoid a correlated subquery for each column I am trying
to update, and instead trying the following, which never returns. There are
2000 records in testnames and about 200 in users.

UPDATE
users
SET
t_firstname = x.firstname,
t_surname = x.lastname,
t_username = x.username,
t_contact = x.email
FROM
(select firstname, lastname, username, email
from testnames order by random()) x
WHERE
t_firstname <> x.firstname;

Any help much appreciated
Rory