I had a similar issue with the T61 laptop. to solve it I wrote a small
appliction to switch the video output, and I looked at acpid how to
configure it to run this app when I press the hotkey.
the application in source code is at
to find out what button your HP has:
# become root
# stop acpid
# look at acpi events
# now press the button and you should see the magic line
# then press ctrl-c to abort the "cat" command above.
# and start acpid again
look at the files in /etc/acpid/events/ and see which file
handles this magic line you have or write a new one.
the files are plain simple: one line "event=" with the magic cookie
and one line "action=" with the command to run.
p.s. maybe give some feedback on that bug report page on launchpad.net
if it works for you, so they can fix your laptop too