squeeze still has no jack support in libportaudio an amd64, ..
.. which is needed to use mixxx with jack
# apt-get source portaudio19
# apt-get build-dep portaudio19
# cd /portaudio19-19+svn20071207/debian
remove ENABLE_JACK = no from line 48
ENABLE_JACK must be explicitely set to "yes"
# nano debian/rules
# dpkg-buildpackage -rfakeroot -b
# cd ../
# dpkg -i *.deb
mixxx still doesn't detect jack for me, stay tuned.
Does work now!
(no) fun! (yet)