If the domain is registered at one of GoDaddys registrars (BlueRazor, GoDaddy, WildWestDomains) than essentially a backorder will act as the first bid when the domain name goes to TDNAM.
Backordering domain names at GoDaddy is not a good choice. The only success they have is with catching .us domain names. Consider snap names uses 30 different registrars to capture expiring domain names, Godaddy only has 1/10 of the capabilities.
GoDaddy also focuses on their own domain names (considering they have the most anyway). Dont bother with a backorder, just wait for the domain to go to TDNAm and a few minutes before the auction ends, place a bid. Dont do it before, you will simply make it easier for other domainers to find that domain names and outbid you.
Justin