Using || operator to fold multiple columns into one

Dear experts,

This point is confusing me with the || operator. I've got a table with
"one column per data type", like so:

# \d context_keyvals;           Table "public.context_keyvals"  Column    |            Type             | Modifiers
-------------+-----------------------------+-----------context_key | integer                     | not nullkeyname     | text                        |t_number    | integer                     |t_string    | text                        |t_boolean   | boolean                     |t_date      | timestamp without time zone |
Indexes:   "context_keyvals_ck" btree (context_key) CLUSTER
Foreign-key constraints:   "context_keyvals_context_key_fkey" FOREIGN KEY (context_key)
REFERENCES contexts(context_key) ON DELETE CASCADE
# select version()
PostgreSQL 8.3.8 on x86_64-pc-linux-gnu, compiled by GCC gcc-4.3.real
(Debian 4.3.2-1.1) 4.3.2

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Just for pretty sake I'd like to be able to use psql to view it like this:

# select context_key,keyname,t_number||t_string||t_date||t_boolean as
value from context_keyvals;

But it is not working, the columns always come up empty.  I can use the
|| operator to concatenate strings:
# select '--'||t_number::text from context_keyvals;

But the moment I try to combine columns, the result is blank.
# select '--'||t_number::text||t_string::text from context_keyvals;

What's up?