BUG #5000: Optimizer does not use function-based index for an order by

The following bug has been logged online:

Bug reference:      5000
Logged by:          Brian Ceccarelli
Email address:      ceccareb@talusmusic.com
PostgreSQL version: 8.2.13
Operating system:   Linux
Description:        Optimizer does not use function-based index for an order
by
Details:

Subject line says it all.
I have a function-based index on a large table.

select *
  from net32_product_def
 order by (f_squash_descr(descr::text))

create unique index net32_product_def_text_idx on net32_product_def (
f_squash_descr(descr) )

The explain plan says that Postgres always does a table scan instead of
using the index.   The table is 500,000 rows long, and the function
f_squash_descr() is a CPU-instensive operation.   What should come
instantly, takes 1 minute.