I found this site by accident today, and I thought it might be useful in decoding some of the RPO Codes that would be listed on ones build sheet. It is a general overall listing of GM's codes (104 pgs). I don't know how up to date or acurate it is. Some things I noticed where on the money. It does take awhile to download for dial up modems, and it is in PDF format.
Sarge
http://www.polariswebdesign.com/GMRPOCodes.pdf