I got mine to work. I manually downloaded an updated apt package from here:
https://packages.debian.org/wheezy/mipsel/apt/download
I transferred the file into my ERX with SCP, then saw that ruh rowh, 100% space used up. Cleared that up with a delete system image (didn't need the RC still in there), then did a sudo dpkg -i apt_0.9.7.9+deb7u7_mipsel.deb.
apt-get worked just fine against http.us.debian.org/wheezy after that. I wonder if the 1.9.0 release will include an updated APT package... hmmm... 