SOLVED! I've managed to set up Ubuntu 10.4 to run pulseaudio over
JACK. But it was a painful process.

Here's what I've found.

- Install pulseaudio-module-jack
- Set your ~/.asoundrc to contain this:

pcm.pulse {
      type pulse
}
ctl.pulse {
      type pulse
}
pcm.!default {
      type pulse
}
ctl.!default {
      type pulse
}

- Create a script ~/bin/startjack with the following contents:

#!/bin/bash
#############################################################
# We want to load pulseaudio AFTER jack so kill the
# current instance, as it will be blocking the audio device.

pulseaudio -k
lsof|grep snd >/home/mrjb/bin/jack.log

# Note: The following line (starting at /usr/bin, ending at -n2)
# is what qjackctl generated to start up jack.

nohup /usr/bin/jackd -dalsa -dhw:0 -r44100 -p1024 -n2 >>/home/mrjb/bin/jack.log &
sleep 1

# restart pulseaudio.

pulseaudio&

# give it some time to come up before trying to load the JACK modules.

sleep 2
pactl load-module module-jack-sink
pactl load-module module-jack-source

# Despite everything, running this script during startup did not
# always seem to work- possibly due to some kind of race
# condition.
# Manually running the script afterwards would invariably
# solve the problem, but defeats the purpose of scripting.
# Because of this, I've added this fugly hack:
# see if startup worked - if not, reinvoke script.
if ps ax | grep -v grep | grep jackd > /dev/null
then
    exit 0
else
   ~/bin/startjack
fi
# start-pulseaudio-x11
########################################################

- I'm calling this as a startup script from
System->Preferences->Startup applications.
- Also, I disabled the regular PulseAudio startup
(which only calls start-pulseaudio-x11 to redirect
system beeps to pretty samples). To guarantee 
the order in which things are loaded, I figured
I could add it to the above script, but in the end
I did not.

- All this and it *STILL* didn't work! It looked like  
pulseaudio -k wasn't killing pulseaudio, as documented.
Turns out, pulseaudio respawns itself by default, so we
need to tell it to stop doing that.

create a file ~/.pulse/client.conf
and add the following, single line in it:

autospawn=no

Now, when running qjackctl, it says "suspending pulseaudio".
I'm not sure why it needs to do that- but I find it annoying
that all audio stops playing when I want to change a JACK 
connection. As it turns out, in Ubuntu qjackctl is inside a 
wrapper script that suspends pulseaudio. The actual qjackctl
is renamed to qjackctl.bin.

Anyway, I changed the relevant snippet of the script
/usr/bin/qjackctl to the following:

        elif [ $PASUSPENDER ]; then
                $QJACKCTL "$@";
                exit 0;
                #echo "Suspending PulseAudio";
                #$PASUSPENDER -- $QJACKCTL "$@";

By default, mplayer uses the default output device which is
alsa - which plays back through pulseaudio which routes to
JACK.

Oh, one more thing- as my soundcard lives in userland,
I want *me* to have access to the device. It's also going
to be *me* starting up pulseaudio anyway.

As such, it's *me* in the audio group, rather than pulseaudio,
just so that it can't grab the device (or so i thought).

Hope this helps someone!

Best,
Marc