Once your are in the terminal you can access all of the terminals without passing through security EXCEPT Terminal 5. Terminal 5 is the international terminal and you have to be holding a ticket or boarding pass to get to the gate areas.
My favorite place inside the terminal at ORD is near the Delta Airlines gates which is at the end of Concourse L.
Most of the international airlines taxi by when the winds are right and the spot is good in the later part of the day when the sun is behind you.
I've got some more info on ORD at:
If you are using Netscape, some of the text might be a little weird...please excuse it. The page is still a work in progress.
Hope this helps
A noble spirit embiggens the smallest man...