constant scalar subselect no longer equivalent to constant?

I upgraded from 7.2.1 to 7.3.2 over the past weekend.  One of my
favorite queries went from an indexed scan to a sequential scan.  Ugh.

Here's the details...  200,000+ records, indexed on "stamp" (a timestamp).
My query was:

    select count(*), otherthing from requests
    where stamp > (select now() - '1 hour' :: interval)
    group by 2
    order by 1 desc
    limit 10;

In 7.2.1, the subselect was treated as a constant, and so the btree
index was used to reduce the scan significantly.  I would get the
result within a few seconds.

In 7.3.2, I get a sequential scan!  Ugh!  It takes about 15 seconds.

However, if I replace the subselect with a constant timestamp
('2003-03-10'), I get an index scan, so it isn't that the analyze
hasn't recognized the number of records.

Did I break something during the upgrade?

Is there anything I can do to get the index scan back?

Is this a known change between 7.2.1 and 7.3.2?

--
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