Tips for identifying an unknown chip?
X-ray won't tell you much.  Dimensions of the chip and where the bonds are attached.  Decap is probably necessary.  You should be able to get the manufacturer from text on the die, which will be a big clue.  If you are really lucky, the manufacturer will put something on the die to help you ID the part number.

If you have a subscription to a tear-down company, you could scan their die photos and if you are luck find a match.

For a fee, a tear-down company will do this work for you and send you a full report on the part. Here's a list of a few such companies:


TechInsights
iSuppli Market Intelligence
Chipworks
Anloy Chip Extraction