Baffled by failure to use index when WHERE uses a function

I have a table cov3 of about 3 million rows, with a B-tree index on an
integer column called hpix.  If I do a simple select on this column it
works in milliseconds, using the index naturally:

select * from cov3 where hpixint = 482787587;
    hpix    |  expos  |  hpixint
-----------+---------+-----------
  482787587 | 30529.6 | 482787587
(1 row)

The problem is that I want to use a user-defined function called healpix
which returns a single integer value in my queries; the function details
are unlikely to be relevant (it selects a pixel from a celestial
position), but its definition is:

  \df healpix
                             List of functions
  Schema |  Name   | Result data type |        Argument data types
--------+---------+------------------+------------------------------------
  public | healpix | integer          | double precision, double precision

So I would like to use this function to find rows, and I try for example:

select * from cov3 where hpix = healpix(2.85,-11.48);

but it takes ages.  An EXPLAIN shows why, it insists upon a sequential
scan:

explain select * from cov3 where hpix = healpix(2.85,-11.48);
                                       QUERY PLAN
--------------------------------------------------------------------------------------
  Seq Scan on cov3  (cost=0.00..93046.81 rows=1 width=20)
    Filter: (hpix = (healpix(2.85::double precision, -11.48::double precision))::text)

Does anyone have any idea why, or know how I can restore adequate
performance?

I am using Postgres 8.1.0 on Linux.

--
Clive Page
Dept of Physics & Astronomy,
University of Leicester,
Leicester, LE1 7RH,  U.K.