What are you backing up; System files?
If you're backing up music, photos or general documents there is no identifying information hidden on those discs. If those files contain personal information, someone can identify you from those files. Personal Information could include comments on music within iTunes, Geo-location and/or faces with iPhoto and names, phone numbers, e-mails within documents.
However, if you burn System Folders for back up, there will be hidden folders and files that may contain revealing information. Regarding your MAC Address, that's hardware specific if I recall correctly. So, if you get a new airport card you'll get a new MAC Address to go with it, though I'm second guessing as I believe my XP partition does have a different MAC than OS X.