Originally Posted by krazykanuck

Maybe true, but where exactly at ORD is there sufficient space for a CL? Not to mention the international and domestic terminals are separate.

It seems that Amex's hit list for where to locate CLs include:

-AA or UA hubs
-Airports where most if not all terminals are connected airside
-Likely significant number of Plat/Cent card holders live in or travel to said city
-High visibility locations that may not fit #1

And then once you have that, as mia mentions, the airport logistically needs to be able to accommodate it. PHL meets 1 and 2, and apparently the airport has the space.