Make sure your system is up to date:
aptitude upgrade aptitude full-upgrade
Now create a testing/unstable mix. Edit /etc/apt/sources.list, duplicate the testing lines (except the security ones) and change the duplicates to sid. Then complete the operation:
echo 'APT::Default-Release "testing";' >> /etc/apt/apt.conf apt-get update
WARNING to Synpatic? users: Synaptic ignores Default-Release. You'll have to also set Preferences → Distribution.
After that, compile and install the Module:
aptitude install module-assistant aptitude -R -t sid install nvidia-kernel-source m-a prepare m-a a-i nvidia aptitude -t sid install nvidia-glx