Curiously enough, the netinst CD does appear to have the missing file...
$ ls -l dists/etch/main/binary-powerpc/ total 688 -rwxr-xr-x 1 rbthomas rbthomas 209980 Dec 8 10:33 Packages -rwxr-xr-x 1 rbthomas rbthomas 131702 Dec 8 10:33 Packages.gz -rwxr-xr-x 1 rbthomas rbthomas 84 Dec 8 10:33 Release