Query planner: current_* vs. explicit date

PsotgreSQL 7.2.4:

Query planner is behaving strangely. It operates differently for explicit dates
and derived dates... any ideas on why? ( and why I might not have noticed this
before... )

CREATE TABLE trans_table (
id serial,
user_name varchar(50),
trans_type varchar(50),
trans_data varchar(50),
trans_date timestamptz,
trans_uuid uniqueidentifier,
CONSTRAINT trans_table_pkey PRIMARY KEY (id)
) WITH OIDS;

... Insert lots of data ...

CREATE INDEX trans_table_date_idx ON trans_table USING btree (trans_date);
CREATE INDEX trans_table_user_date_idx ON trans_table USING btree
(user_name,trans_date);
CREATE INDEX trans_table_uuid_idx ON trans_table USING btree (trans_uuid);

VACUUM ANALYZE trans_table;

EXPLAIN SELECT id FROM trans_table WHERE trans_date >= current_date::timestamp;
Seq Scan on trans_table (cost=0.00..177369.52 rows=315267 width=4)

EXPLAIN SELECT id FROM trans_table WHERE trans_date >= '10/22/2003 00:00:00
AM'::timestamp;
Index Scan using trans_table_date_idx on trans_table (cost=0.00..1474.69
rows=417 width=4)

CG

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