Does Red Hat Linux include an X Window system?

Yes, you can customize the installation with your requirement.

Indeed X windows included in RHEL

Indeed it is, during setup you can select the X Window system packages of your choice.

The default RHEL6 install wont have the X Window (unlike the RHEL/CENTOS5)

X windows is included in the package from redhat3

it is not a part of default install in some varients of RHEL, we have cutomize software package instllation for adding X sever. 

Yes you can start that at run level-5 by issuing startx command.

YES you can. But select it during installation. you can also install after installation but its good to install during installation.  

Sure. But dont forget to install it.

X Window is not  a part of Linux by default.It is a different package.If you download linux from internet may be you won't get X window.

But if you buy DVD in most cases you will find X Window in that DVD.Because they add some extra packages in that DVD.