As already mentioned on <firstname.lastname@example.org> libvirt 0.7.4-2 (currently in experimental) runs qemu and kvm processes as libvirt-qemu:kvm instead of root allowing you to use features like bridged networking without having to run the emulator itself with full privileges.
echo 1 > /sys/kernel/mm/ksm/run
to activate it. Then run two similar virtual machines and see how /sys/kernel/mm/ksm/pages_shared increases.
Thanks to work of Jan and Micheal Debian now also ships kvm's stable branch qemu-kvm.