Concatenating several rows

["Pierre Thibaudeau" <pierdeux(at)gmail(dot)com>]

SELECT name FROM pseudonyms WHERE person_id=125;

I know in advance that this query yields between 0 and 5 rows of
results, depending on the actual person_id.

How can I concatenate those results into one text string?  Something like:

SELECT array_to_string( (SELECT name FROM pseudonyms WHERE person_id=125), ' ');

which doesn't work...