It's the sticker with the serial number on it:
http://docs.info.apple.com/article.html?artnum=88138
Actually, I believe there are two stickers: one with the serial number and one with the Ethernet i.d. number. You will need to remove both stickers, but put them in a safe place, and I would also recommend keeping track of them in a text file or Sticky Note on your computer. Be sure to remove all traces of glue, also, in case it turns out to be the smelly culprit. Don't use strong solvents that could melt the plastic. A tiny bit of Pledge dusting spray sprayed on the corner of a clean soft cloth may be helpful in removing the adhesive.
Be sure to shut down and remove the battery before removing the keyboard.
Good luck, and let us know if that was the source of your problem.