However, I am still unclear about how to access the contents of StreamingAssets.
Suppose I have something like /Assets/StreamingAssets/Config/config.xml,
in what loacation will I see it inside the .apk on the device?
With Unity pre-3.2 the StreamingAssets was intended for streaming data (like video) only. The reason is that the .apk is never decompiled when installing it, so any file inside the .apk would have to be read through a zip-reader (.apks being regular .zip files, with a manifest on top).
The Asset-bundles-in-the-data-folder thread describes how the WWW API can be used as a zip-reader in Unity 3.2 (still in beta though)