I don't know if there ever actually (was) an iso image at: http://cdimage.debian.org/debian-cd/3.1_r5/amd64/iso-cd/debian-31r5-amd64-netinst.iso but I am fairly certain that the isn't now. Maybe we could simply link this shortcut (on http://www.debian.org/CD/netinst/) to the (unofficial) amd64 netinstall cd iso image located at: http://cdimage.debian.org/cdimage/unofficial/sarge-amd64/iso-cd/debian-31r4-amd64-netinst.iso
I don't know if I should've submitted this here, or in bug reports, so if it should have been submitted elsewhere, please forward it to the proper location. I'll send only this message on the topic so as not to create wasted/confusing multiple instances of a problem report. Thank You, Robert Hercules (returning) linux convert