Not all CDs have UPCs, and even Amazon doesn't use those to identify their products (they use an ASN). The CDDB uses the length of the tracks to guess which CD you have, but sometimes that even comes up with more than one possibility. There are a couple of Perl modules for that too, but you have to come up with the CD info. A combination of those should identity a disk in most of the cases however.

I don't pay much attention to either of those: I put the CDs into the computer, let the music palyer figure it out (using CDDB), then look at the stuff the music player stores. For some of them, you don't even need to rip the disk: it remembers what it has already looked up. :)