Have successfully installed iPC 10.5.6 on Thinkpad X300. Considerably easier and elegant than other X86 installations.

-Start with a X300 with an Atheros wifi-card with suitably flashed bios.
-Boot using iPC 10.5.6 installation dvd
-Partition SSD using GUID option
-Choose following options:
iPC OsX86 base
language kit, if needed
AD1984 sound
Intel 82566MM/DC ethernet driver
IO80211Family.kext for wifi
PCGenUSBEHCI for usb
PS/2 Keyboard fix
DSDT Patch
AppleSMBIOS-27 Rev6 Macbook
from application software:
cpu-x (gives CPU info)
kext helper b7 (useful for installing kext)
OsX86 tool utility (useful for archiving/restoring system extensions)
DSDT Patcher GUI

Reboot and will get welcome graphics with screen that is not properly scaled so after the first reboot, boot with -s and type:
mount -uw /
rm -rf /System/Library/Extensions/AppleIntelGMA*
rm -rf /System/Library/Extensions/AppleIntelIntelligentFramebuffer.kext
reboot

This will reboot the X300 into default 1024x768 resolution. You will get a working ethernet and wifi right away. There is a X3100 solution proposed by Slashack on http://macyourpc.com/2009/03/20/full-co ... 0-screens/
Download and install the X3100 kexts using kext helper. Reboot and will get fully working 1440x900 CI/QE. (Brightness and video mirror are still problematic.)

You can install EnhancedSpeedStep or Voodoo Power by Superhai to reduce the CPU heating. You can also install Temperature Monitor to see the temperature of two cores (generally below 50 degrees Celcius on my x300.)

If you are brave, you can even download and install the 10.5.7 updater. Before you do that, it will be wiser to use OsX86 tool utility to archive the kexts from 10.5.6. 10.5.7 updater will run without crashing except it will likely ruin the X3100 graphics. Reinstall the X3100 graphics kexts again and viola you will have a working 10.5.7 system.

Alternatively you can restore the kexts from 10.5.6 but there may be unforeseen issues that I haven't chased after. I have found that USB in 10.5.7 no longer works so some of the kexts from 10.5.6 need to be restored. There may be other problematic issues but I've elected to wait until 10.5.7 updates get flushed further.

In summary, iPC 10.5.6 gives rather stable x86 without much effort. Working ethernet and wifi are big pluses. It is possible to get fully working CI/QE on x3100 though sleep, brightness, and video mirroring are still problematic but this will not prevent from a working hackpad. DSDT patching is supposed to give a measure of brightness control though I haven't tried to work on it yet. YMMV.

Do you get support for second display?
What does 'problematic' for mirror-mode mean?
How well do second display and mirror mode work, what does not?

Yes, second LCD will work but only in the extended mode. VGA is pretty limited so I don't bother with this.

Turning on the video mirror will scramble your main LCD and the external monitor.

Connecting a projector is even worse and will require a reboot after a hardware kill as X300 will not recover well from having a projector connected.

installation of 10.5.8 (combined) update to an iPC 10.5.6 on X300 works without any apparent problem - other than seemingly taking a long time for the update to finish.

only hiccup is trashing of video on LCD but reinstallation of Slashack's X3100 after removal of AppleIntelGMA* and AppleIntelIngelligentFramebuffer as described above restores QE/CI.

even the security update seems to install ok; caution on updates of Safari 4.0.3 and Java 4 update. some members of insanelymac report problems after installation of updates though it wasn't on a X300;

safari 4.0.3 seems to cause a number of problems in non-leo/non-intel macs for some reason; probably better to wait 4.0.4 or higher before updating; I am not yet ready to install Java 4 update but be interested in learning what others have experienced...;there is an increasing interest in installation of Snow Leopard 10.6 rather than 10.5.8 in X86 community

some issues with voodoopower is reported as well as 10.5.8 runs cooler without speedstep, though I can't say anything about 10.5.8 effect on battery performance for now; voodoobattery seems to work fine