Creating then dropping primary key constraint

Hi,

I noticed that when I create a primary key with ALTER TABLE ... ADD
CONSTRAINT ... PRIMARY KEY (...),
and then drop this constraint, then the "not null" modifier stays on the
column on which the primary key was defined although there were no
constraint on that column before.
Is this normal?
Pg 8.0.4

create table pritest(id integer);
\d pritest
alter table pritest add constraint pk_pritest primary key (id);
\d pritest
alter table pritest drop constraint pk_pritest;
\d pritest
drop table pritest;

Best regards,
Otto