It looks like the serial number is in the SetupInfo.plist file, but it's obfuscated in some way.
EDIT: The following comes from here:
Is there an easy way to do trial and
error and change the key if it was
already used?
If you look in the Application menu
for the About item and display that,
it will display the Product ID to you.
If you find two machines on which all
but the last five digits match, the
same key was used on those two
machines.
It won't tell you "which" key was
used, just that the "same" key was
used.
Only trial and error will let you sort
out which key belongs to which
computer. You can change the licenses
without removing and reinstalling
however.
Easiest method: Launch the Remove
Office tool and hold down Option at
the first dialog, and the "remove
office" option will change to "remove
licensing information".
/Application/Microsoft Office
2008/Additional Tools/Remove Office
If you have the 3 systems connected,
start one of the Office apps on the
oldest of the bunch (IIRC the product
keys are in a list so you probably
used the top one on that system.).
Then launch the same app on one of the
other two. If there is a conflict
modify the second one's product key.
If not the 3rd one must be the odd man
out - although you may still have to
get 2 & 3 to bump heads in order to be
certain.
