After upgrade to xf86-video-intel 2.10.0 required turning on KMS in kernel, I got a blank screen after reboot. Logging in blind and starting X, my dual screen setup duplicated the output on the two screens instead of keeping them separate, and the resolution was reduced to 1023x768. This was on 2.6.32-tuxonice-r1. I found the following bug report that seemed relevant:
However, I tried downgrading to 2.6.31-tuxonice, but the behavior was unchanged.
Steps to Reproduce:
1.enable KMS per the instructions in the xf86-video-intel 2.10.0 note
After boot process starts, screen goes blank and remains blank. Logging in blind it is possible to start X, but the screen resolution is wrong and the dual screen setup does not work correctly.
Screen remains active, X works normally.
00:02.0 VGA compatible controller: Intel Corporation Mobile 945GM/GMS, 943/940GML Express Integrated Graphics Controller (rev 03)
I have another machine with an Intel video card:
00:02.0 VGA compatible controller: Intel Corporation 82G965 Integrated Graphics Controller (rev 02)
that shows the same blank-screen symptom on boot. It is running kernel 2.6.33:
Linux maestro 2.6.33-gentoo #2 SMP PREEMPT Sat Mar 20 17:55:48 EDT 2010 i686 Intel(R) Core(TM)2 CPU 6600 @ 2.40GHz GenuineIntel GNU/Linux
If I log in blind on this machine (which has only one screen), X works fine.
This is beyond my knowledge of the driver. Please file a bug upstream  and paste the URL here so I can track the bug too.
I found the answer in a thread on the arch linux forum. It turns out that you have to remove any 'video=' parameters from the kernel boot line, and then everything works fine. And my X problem turned out to be that the names of the devices had changed, and so my script that sets up my dual screen wasn't doing anything any more because it was using the wrong names.