case-sensitivity and the serial type

I've been using the debian package of postgresql (7.0-release-1)
and I've been able to say something like:

create table blah ( id serial );

Note how serial is all lowercase. I have since compiled
postgresql 7.0.2 on my friends box and installed it in my
home directory so I could play w/ it. I was surprised when
I couldn't build my table w/ the above statement, but if I
were to do:

create table blah ( id SERIAL );

(note how serial is all uppercase, now) it works.

Can anyone explain why this happens? I guess for the time
being, I'll capitalize every instance of 'serial' when I
create tables, but this just got me curious.

thanks!