Intel DH55TC motherboard

Karmic can be installed, but some quirks are required.
See https://bugs.launchpad.net/ubuntu/+source/linux/+bug/507770
Lucid is not affected by this problem, and installs fine.
Overall an excellent board for Linux, all hardware is detected and work out of the box.
Highly suggested as a fast, stable and all-round system with support from a good brand.
Definitely not a board for over-clockers due to lack of cpu or memory voltage override option. It is still possible to override bus frequency though, and with BIOS version 31 it is also possible to change memory timings.
Apparently a memory voltage override option might be added in a future revision of the board.

GRAPHICS

2D and Compiz effects work well. Tested with 1920x1080 display connected through DVI.
The screen can occasionally freeze: see bug #560433, there are 2 possible solutions:
1. disable compiz visual effects
2. upgrade to 2.6.34 from http://kernel.ubuntu.com/~kernel-ppa/mainline/
Direct rendering works OK: tested with Warzone 2100 and Quake 3.
Quake 3 has very playable frame-rates at 1920x1080 with maximum settings. Doom 3 and Quake Wars Enemy Territory crash during start-up with the Lucid stock kernel. With the 2.6.34 kernel QWET makes it as far as the menu, but crashes when game starts. Doom 3 makes it as far as asking for the key, but I don't have a valid one =)
Dual screen works fine, tested with DVI and VGA.

SOUND

The rear analogue sound output (the green jack) works fine.
Left to be tested are: the front panel analogue header, the SPDIF header, and the HDMI sound output.

NETWORK CARD

Just works.
There are reports that when running 2.6.33 the latest version of the driver from Intel has to be used for networking to work.

ACPI

With an i5 660 Speedstep the speed is scaled in 9 steps from 1.2 to 3.33 GHz.
The module 'coretemp' works fine even though is not inserted by default by Ubuntu and displays the temperatures of both cores using the sensors command line utility.
The module w83627ehf displays 2 temperatures that are plausible, and raise when the system is loaded, but voltages and fan figures are wrong or missing.
The Intel QST SDK released by Intel in February 2010 looks very promising and compiles fine, except that it requires a HECI kernel module, that can be downloaded from www.openamt.org but is hopelessly outdated and doesn't work on any H55 board.
Suspend/resume from S3 works fine except for the network card that doesn't seem to come back to life.

BIOS SETTINGS

Hyperthreading: works
Turbo boost: works (see below)
Virtualization: works, virtualbox detected VT-x
VT-d: not tested. Software support for this new extension seems to be non-existent at present.

RAM

Memory can only be set to 1.5v so running one of those heat-spreader fitted modules publicized as having good timing like 7-7-7 at 1.65v is highly unadvisable. These memories will likely produce error at 1.5v even when lowering their frequency/timings.
Modules reported as compatible in Intel website will likely produce best results.
Kingston KVR1333d3n9k2/4g is reported to work very well with this board.

TURBO BOOST

It seems to work fine, even if you will not find out by looking at /proc/cpuinfo or any of the cpufreq entries in /sys !
To check if it really is working, you can try this utility: http://code.google.com/p/i7z/