I am using Gentoo Linux, and starting with Linux 2.6.39, I cannot start X if I have an external monitor connected.

I originally reported this bug to Gentoo, and they asked me to report it upstream. The original report is here: https://bugs.gentoo.org/show_bug.cgi?id=378555

I've copied some relevant info from that ticket here:

Unfortunately,
because it appears to be a kernel panic, and because X blanks the screen, I am
unable to see the panic message itself, and also nothing appears in the log,
which makes this a very difficult problem to search for to determine if it has
already been reported.

If I disable the xdm init script, the machine boots to the console just fine.
Also, if I do not have any external monitor connected, X will start and operate
just fine. If I add the external monitor after I am already logged in to KDE,
the second monitor will operate correctly.

I have been able to reproduce this problem with two different external monitors
to verify that it isn't the particular monitor I am connecting.

gentoo-sources-2.6.38 does not suffer from this problem.

Reproducible: Always

Steps to Reproduce:
1. Connect an external monitor
2. Boot the machine, with xdm enabled, or start XDM.
Actual Results:  
The laptop's screen appears to turn off (backlight off). The external monitor
is on, with a non-blinking cursor appearing in the upper left corner of the
screen. At this point, the machine appears to be completely frozen, not
responding to ping requests. I am also not able to get to any of the virtual
terminals.

Expected Results:  
The machine should boot into KDM.

I am currently using x11-drivers/xf86-video-intel-2.15.0-r1 (the failure also
happens with 2.14), and sys-kernel/gentoo-sources-3.0.0 (also happens with
2.6.39).

I do not have an xorg.conf at all. Here is lspci output:

# lspci
00:00.0 Host bridge: Intel Corporation Core Processor DRAM Controller (rev 02)
00:02.0 VGA compatible controller: Intel Corporation Core Processor Integrated
Graphics Controller (rev 02)
00:19.0 Ethernet controller: Intel Corporation 82577LM Gigabit Network
Connection (rev 05)
00:1a.0 USB Controller: Intel Corporation 5 Series/3400 Series Chipset USB2
Enhanced Host Controller (rev 05)
00:1b.0 Audio device: Intel Corporation 5 Series/3400 Series Chipset High
Definition Audio (rev 05)
00:1c.0 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express
Root Port 1 (rev 05)
00:1c.1 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express
Root Port 2 (rev 05)
00:1c.2 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express
Root Port 3 (rev 05)
00:1c.3 PCI bridge: Intel Corporation 5 Series/3400 Series Chipset PCI Express
Root Port 4 (rev 05)
00:1d.0 USB Controller: Intel Corporation 5 Series/3400 Series Chipset USB2
Enhanced Host Controller (rev 05)
00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev a5)
00:1f.0 ISA bridge: Intel Corporation Mobile 5 Series Chipset LPC Interface
Controller (rev 05)
00:1f.2 RAID bus controller: Intel Corporation Mobile 82801 SATA RAID
Controller (rev 05)
00:1f.3 SMBus: Intel Corporation 5 Series/3400 Series Chipset SMBus Controller
(rev 05)
00:1f.6 Signal processing controller: Intel Corporation 5 Series/3400 Series
Chipset Thermal Subsystem (rev 05)
02:00.0 Network controller: Broadcom Corporation Device 4727 (rev 01)
03:00.0 CardBus bridge: Ricoh Co Ltd Device e476 (rev 02)
03:00.1 SD Host controller: Ricoh Co Ltd Device e822 (rev 03)
03:00.4 FireWire (IEEE 1394): Ricoh Co Ltd Device e832 (rev 03)
3f:00.0 Host bridge: Intel Corporation Core Processor QuickPath Architecture
Generic Non-core Registers (rev 02)
3f:00.1 Host bridge: Intel Corporation Core Processor QuickPath Architecture
System Address Decoder (rev 02)
3f:02.0 Host bridge: Intel Corporation Core Processor QPI Link 0 (rev 02)
3f:02.1 Host bridge: Intel Corporation Core Processor QPI Physical 0 (rev 02)
3f:02.2 Host bridge: Intel Corporation Core Processor Reserved (rev 02)
3f:02.3 Host bridge: Intel Corporation Core Processor Reserved (rev 02)

This is a Dell E6510.