Bug#374334: "failed to determine the codename for this release"
In my case (net iso of 22 dec 2009) this was a file marker, i.e.
rename/remove it (since we want to get files from outside), e.g.
$ mv /cdrom/.disk/base_installable /cdrom/.disk/base_installable.false
in that case mirror settings will be used to retrieve components (debs) instead of cdrom itself.
Hope this help.