indexing a datetime by date

I have a table with a field, "when", of type "datetime".  I can't use
"date" because I need the times as well.  I'm using PostgreSQL 6.4.2.

I'd like to identify all of the records with today's date, as in:

select when from notes where when::date = now()::date;

The query works, but is very slow.  Explain confirms that a sequential
scan is being used.

I've tried indexing on when:

create index when_ndx1 on notes (when);

But that doesn't help, as (I suppose) the optimizer can't match
when::date with this index.

Neither of these works:

db=> create index when_ndx2 on notes (when::date);
ERROR:  parser: parse error at or near "::"

db=> create index when_ndx3 on notes (date(when));
ERROR:  DefineIndex:  class not found

As a workaround, I've been using this:

select when from notes where when >= '3/29/1999 0:0:0' and when <=
'3/29/1999 23:59:59';

but that's ugly and requires hardcoding today's date each time, rather
than using now().

So, the question is, is there a way to index a datetime field by date?

Andrew Merrill