Baffled by failure to use index when WHERE uses a function

[Clive Page <cgp@xxxxxxxxxxxxx>]

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.