UPDATE - due to an unrelated xfce issue I rediscovered a way to get a 'minimal' LM12-XFCE system.
- Download and install base Ubuntu 11.10 Oneiric from the 32 or 64 bit mini.iso - it takes longer than a CD/ISO install because drivers and base pkgs have to be downloaded. You will reboot into a command-line only system.
- Using apt-get or aptitude, install xfce4, xfce4-session, lightdm and any other utils that you like (say, synaptic).
- Add the LM12 'lisa' repo to the beginning of /etc/apt/sources.list, refresh and add any Mint metapackages that you like.
You don't need the xfce4-goodies or xubuntu-defaults unless you like them . Really.
Starting with the LM12 LXDE spin will also get you a pretty lightweight install, but it is 32 bit only at this time.