indexes using datetime

Hi.
I've got a slow query I can't seem to fix...
Table    = logins
+-------------------------+----------------------------------+-------+
|           Field         |              Type                | Length|
+-------------------------+----------------------------------+-------+
| loginid                 | varchar()                        |    16 |
| logintime               | datetime                         |     8 |
| ip                      | varchar()                        |    15 |
| direction               | char()                           |     1 |
+-------------------------+----------------------------------+-------+
Indices:  logins_direction_idx         logins_logintime_idx

So I've got this table that records stuff about people logging in. Since
it's essentially a log, it is very large. About 1.3 million tuples.

I've created an index on the logintime with hopes I can make some of my
queries which care only about the last 40 day's of logins use the query
and ignore the rest of the tuples...

explain select * from logins where logintime>'now'::datetime-'40
days'::timespan;
NOTICE:  QUERY PLAN:

Seq Scan on logins  (cost=5839.78 rows=44958 width=44)

Very bad query plan :(

As I recall, you should be able to make a btree traversal return all its
values in order. So then isn't it reasonable that the dbms should be using
the index to find all the values within a certain range?

-Michael