Comment 3 for bug 751453

I think I've figured this out.

In my BIOS (hit esc and/or f1 repeatedly at boot to get to it), there's an
option "Power on Display". It's not visible on the first page; I had to hit
page down to bring up the second page.

My laptop was set to "auto-selected", if I switch it to the only other option
"LCD+Analog RGB", then I can hot-plug an external display at run time in Linux,
regardless of whether I booted with one attached.

Note that I still think there's a bug in Linux here: if the ext display is not powered,
but still detected, it should power it up. I think Windows gets this right, but I have
to confirm that.