Keep in mind that I downloaded my copy from MSDN, so mine had my key embedded already. I'm not sure whether or not this will be the case with physical media.
Look in ExpressionStudioManifest.cab, you should find a single file called ExpressionStudioManifest.xml. Do a search in that for PROOFOFPURCHASE. The value of that entry is your Key.
You can then simply apply that key to any MSI installation by using the PROOFOFPURCHASE property, either via command line or MST. The reason I missed it before was I was looking for the older property, PIDKEY.
For example, to install only Web Expressions 3, my command line looks like this: