[I suggest to discuss the matter on the debian-powerpc list, since I've not been able to reproduce the problem on i386.]
Trying to install Debian on a PowerBook G3, with boot-floppies 2.2.13, I noticed that retrieving any file by HTTP yields an "error installing file drivers.tgz" (or base2_2.tgz. "zcat THEFILE | tar tvf -" shows that it is indeed corrupted. Using an HTTP proxy or not changes nothing. On another Linux machine (a i386), I can retrieve the very same file with HTTP clients and there is no corruption. Exporting then this file with NFS, I was able to complete the installation. So, it really seems HTTP transfer in dbootstrap corrupt files. Did anyone use it with success?