Null integer columns - Mailing list pgsql-performance

I have an integer column that is not needed for some rows in the table
(whether it is necessary is a factor of other row attributes, and it
isn't worth putting in a separate table).

What are the performance tradeoffs (storage space, query speed) of using
a NULL enabled column versus a NOT-NULL column with a sentinel integer
value?

Not that it matters, but in the event where the column values matter,
the numberic value is a foreign key.  Advice on that welcome too.

Thanks!