Choice of bitmap scan over index scan

Hi,

Part of a larger problem, I'm trying to optimize a rather simple query which
is basically:
SELECT * FROM table WHERE indexed_column > ... ORDER BY indexed_column DESC;

(see attachment for all details: table definition, query, query plans)

For small ranges it will choose an index scan which is very good. For
somewhat larger ranges (not very large yet) it will switch to a bitmap scan
+ sorting. Pgsql probably thinks that the larger the range, the better a
bitmap scan is because it reads more effectively. However, in my case, the
larger the query, the worse bitmap+sort performs compared to index scan:

Small range (5K rows): 5.4 msec (b+s) vs 3.3 msec (i) -- performance penalty
of ~50%
Large range (1.5M rows): 6400 sec (b+s) vs 2100 msec (i) -- performance
penalty of ~200%

How can I make pgsql realize that it should always pick the index scan?

Thanks!

Kind regards,
Mathieu