Bitmap Heap scan 8.1/8.2

I have to PG servers, one ver. 8.1.9 and the other 8.2.4.

I was checking a query out and found that with the exact same DB (same
data in it) and the same query I get different plans, and significantly
higher time in 8.2:

On 8.1 I get:

test=> explain analyze SELECT * FROM prestamos WHERE biblioteca = 19 AND
  vencimiento < now() AND NOT devuelto ORDER BY vencimiento DESC;

QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------------------
  Sort  (cost=2793.74..2795.82 rows=832 width=20) (actual
time=25.795..25.832 rows=49 loops=1)
    Sort Key: vencimiento
    ->  Bitmap Heap Scan on prestamos  (cost=850.43..2753.39 rows=832
width=20) (actual time=20.747..25.529 rows=49 loops=1)
          Filter: ((biblioteca = 19) AND (vencimiento < now()) AND (NOT
devuelto))
          ->  Bitmap Index Scan on prestamos_objetos_devuelto_idx
(cost=0.00..850.43 rows=2200 width=0) (actual time=20.265..20.265
rows=2301 loops=1)
                Index Cond: (devuelto = false)
  Total runtime: 25.971 ms
(7 filas)

On 8.2:

test=> explain analyze SELECT * FROM prestamos WHERE biblioteca = 19 AND
  vencimiento < now() AND NOT devuelto ORDER BY vencimiento DESC;
                                                    QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
  Sort  (cost=4100.77..4102.77 rows=800 width=20) (actual
time=95.082..95.103 rows=49 loops=1)
    Sort Key: vencimiento
    ->  Seq Scan on prestamos  (cost=0.00..4062.20 rows=800 width=20)
(actual time=7.293..82.778 rows=49 loops=1)
          Filter: ((biblioteca = 19) AND (vencimiento < now()) AND (NOT
devuelto))
  Total runtime: 95.165 ms
(5 filas)

It's like it's ignoring the HEAP scan. Why? It's obviously (unless I'm
totally wrong) faster in this case.

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