alter table performance

Hi,

I am regularly altering tables, adding columns setting default values etc.
This very often takes a very long time and is very disk intensive, and this
gets pretty annoying.

Things are hampered by the fact that some of our servers run PG 7.3

Suppose I have a table and I want to add a non NULL column with a default value.
What I normally do is:

alter table person add column address varchar(64);
update person set address = '' where address IS NULL;
alter table person alter column address set not NULL;
alter table person alter column address set default '';

When the table contains millions of records this takes forever.

Am I doing something wrong? Do other people have the same problems?

Thanks,
Antonio