Downgrade database and problem with sequences

I am currently using postgresql 8.3 and whenever I want to get the last
inserted ID use :
Select currval(pg_get_serial_sequence(table, field))

Then, I had to downgrade a database for certain customer to use postgresql
8.1, and when I execute that same function I found that
pg_get_serial_sequence returns nothing.

Is there any way to dump my database and then restore it on a previous
version and keep using that pg_get_serial_sequence()  function ? 

Thanks,

Sebastian