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)