I don't currently have enough reputation to comment on the answer above (in fact, I've signed up just to comment). Supremely helpful!
Two elaborations: I was installing the resulting file system on my Thecus 4100+ NAS box, and the current Wheezy Raspbian distribution is on an ext4 format .img file, which my NAS doesn't support (it's too old, and understands ext3). So I had to do the extraction on my current Linux server. I originally tried doing the extraction with the cp -rav command listed above, writting directly to an NFS mount from the NAS box, but couldn't boot the from that, due to issues over the file permissions. I could probably have dealt with that via NFS, but in the end, I created a tar file directly from the loopback file system, which I then untarred on the NAS box.
secondly (and very minor): according to the rcS man page:
The RAMLOCK, RAMSHM and RAMTMP variables have been moved to /etc/default/tmpfs; RAMSHM and RAMTMP settings in rcS are used (if set) for backward compatibility, but will be overridden by settings enabled in /etc/default/tmpfs.
(I also had the rootdev issue, which originates in the /etc/init.d/checkroot.sh script. I resolved it by altering /etc/fstab as Klaudiusz suggests.)