Now I've actually found *2* of these things:
P3: DIL20 header with one pin missing (polariser)
P4: DIL14 header with one pin missing (polariser)
Both are individually marked "LCD", so what's the difference, if any?

That's curious. I have two of these IMD's here in front of me. One is
out of a 2500, and the other is out of a 3000. They both have a single
34 pin (with one pin blocked out) ribbon cable. I am hoping that the
20pin and 14pin headers that you found are directly side by side,
making one 34pin header. If this is the case, then you should have no
trouble.

Now... what does this give you?

The LCD will be lit and functioning as long as the power cable is
plugged in...even with the system powered down. During boot up, it will
echo all of the posting steps (CPU initialization, memory test, etc.)
even before the display is initialized. After bootup it will provide
you with quick easy access to any error messages incurred during boot
(the arrow keys will navigate through the list). During normal
operation, you can install the Integrated Management Display Utility,
from the ProLiant Support Pack, and setup a custom default display of
your choice. My 3000's are setup to display this information during
normal running:

Line 1) System Name
Line 2) System Time
Line 3) CPU Usage (a bar graph and %)
Line 4) Main Menu

Of course there are many other options. You may wish to display a graph
of memory usage, for example.

If the server experience a catastrophic failure, you will get a readout
of what happened (as long as the power cord is plugged in and hot)
enabling you to take action before you power up the system again.

it would be nice to see this the easy way - just plug in a display and let 'er rip.

Just remove the power cord, plug the display into the header, and
reinsert the power cord. If you immediately see the backlight and the
words "Compaq ProLiant 1600", then the ripping has begun!

Let me know if you need of of my spare IMD's...I'm sure we can work
something out better than ebay's shipping charges.

Jeff