index not used with subselect in where clause ?

Good evening (GMT+0200) everybody...

I have a table with an indexed integer attribute used to
determine which rows to fetch from my data base.
(pgsql version is 7.1RC4)

When doing a

SELECT attr1 FROM table1 WHERE attr1 IN (<list of ints>)
AND <more conditions>;

where <list of ints> is entered explicitly (or generated by a
program) everything works fine and fast using the index on attr1.

But when I try doing a

SELECT attr1 FROM table1 WHERE attr1 IN (<SUBSELECT returning list of ints>)
AND <more conditions>;

then the SELECT on table1 uses a sequential scan running 'endlessly'.

Is this really how it's meant to be?

If so, why?
If not, how can I get the second version working???

Thanks in advance...
Christian

--
"The sky above the port was the color of television,
tuned to a dead channel."
-- W.G. --