Problem with text_pattern_ops

I have this index:

"directory_lower_username_seg_key" unique, btree (lower(username)
text_pattern_ops, seg)

... but my query refuses to use that index.
[local]:owl=>explain select * from directory where lower(username) =
'jks@selectacast.net';
                           QUERY PLAN
---------------------------------------------------------------
  Seq Scan on directory  (cost=0.00..860.75 rows=102 width=575)
    Filter: (lower(username) = 'jks@selectacast.net'::text)
(2 rows)
[local]:owl=>CREATE INDEX directory_lower_username_seg_key1 ON directory
(lower(username),seg);
CREATE INDEX
[local]:owl=>explain select * from directory where lower(username) =
'jks@selectacast.net';
                                                QUERY PLAN
---------------------------------------------------------------------------------------------------------
  Index Scan using directory_lower_username_seg_key1 on directory
(cost=0.00..377.01 rows=102 width=575)
    Index Cond: (lower(username) = 'jks@selectacast.net'::text)
(2 rows)

[local]:owl=>select version();
                                                  version
---------------------------------------------------------------------------------------------------------
  PostgreSQL 7.4.7 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2.2
20030222 (Red Hat Linux 3.2.2-5)
(1 row)


So what gives?