Continuing the trend of accidentally disabling hacks that people rely on, it now no longer appears to be possible to set your network card's MAC by adding a line like the following to /etc/rc.local:

ifconfig eth0 hw ether 00:0C:xx:xx:xx:xx

It seems that for some reason something (presumably NetworkManager) is resetting the device's MAC address when the computer wakes from suspend. However, at the same time, the "Cloned MAC address" field has been added to NetworkManager's "Edit connection" dialog, and this is where your desired MAC address should be entered.