index not always used when selecting on a date field

I have a database with a btree index on the 'removed' field,
which is of type 'date'. However it isn't being used:

techdb2=> explain select * from lines where removed > CURRENT_DATE;
QUERY PLAN
------------------------------------------------------------
Seq Scan on lines (cost=0.00..243.47 rows=2189 width=324)
Filter: (removed > ('now'::text)::date)
(2 rows)

Now the weird thing is that if I select a range it is being used:

techdb2=> explain select * from lines where removed > CURRENT_DATE and removed < '9999-01-01';
QUERY PLAN
------------------------------------------------------------------------------------
Index Scan using lines_removed_idx on lines (cost=0.00..120.56 rows=33 width=324)
Index Cond: ((removed > ('now'::text)::date) AND (removed < '9999-01-01'::date))
(2 rows)

Why is this?

(Tested with both 7.3.2 and 7.4.6)

Mike.