B-tree index with sorting question

In the docs at http://www.postgresql.org/docs/9.0/static/sql-createindex.html,
I see that you can build indexes that include ordering.
Eg. create index t_col_idx on t (col DESC NULLS LAST);

Does that mean that the initial creation of the index acts like the
CLUSTER command? If so, what happens to subsequent inserts/updates of
the indexed attribute? Are those values then indexed in the order that
they are inserted -- which could be contrary to the ordering specified
in the index creation?