using a sequence as the functional equivalent to Oracle rownum

I'm trying to use a temporary sequence to duplicate the functionality of
the Oracle rownum pseudo-column
as suggested by Scott Marlow in the archives:
http://archives.postgresql.org/pgsql-sql/2005-05/msg00126.php.

The Oracle based application I'm porting to PostgreSQL used rownum to
select the 'next' block of rows to
process by specifying a where clause with something like " where
rownum>x and rownum<y "

My basic PostgreSQL query is:

drop sequence rownum ;
create temp sequence rownum;

select B.rownum , B.id from
(select nextval('rownum') as rownum, A.* from
(select distinct id from ... where ... order by ... DESC
) as A
) as B
where id>0

This basic query produces the following result set:
rownum id
--------+---------
1 10038
2 10809
3 10810
4 22549
5 23023

However, if I add a where clause referencing rownum for example: where
id>0 and rownum>0
I get the following:

rownum id
-------+---------
11 10038
12 10809
13 10810
14 22549
15 23023

It appears as if rownum has been incremented as a result of three passes
over the five row result set.

Can someone explain what is going on? And more to to point, if this is
expected behavior, is there a standard PostgreSQL way to select a
'block' of rows from a result set based on row number?

Thanks,
-=bill