Last I heard the ALSA drivers did mux the sound out on HDMI, but I have not actually tried it myself, so I'll concede it may be broken on the Linux side.

All Intel G965 motherboards
Shuttle SG33G5
Gigabyte GA-G33M-S2H
Asus P5E-V HDMI
Asus P5E-VM HDMI

"Like who?"

Asus, Auzentech

"What software works?"

Mediaportal, Media Player Classic-- either with FFDshow LPCM filters
ZoomPlayer probably works too

i.e. "full resolution 6- and 8-channel audio simply cannot be passed digitally in any other way than on certain specific Intel platforms"

Which ones?

"there are a couple of sound card manufacturers who are now working on releasing a "passthrough", where you feed your HDMI video from a video card into your soundcard, which adds the audio and then resends it all out over another HDMI port"

Like who?

"Right now, there are so few audio solutions which accomodate this that all playback software"

What software works?

Altrought Im not huge fan of intel and their business strategies I can admit that this is exelent article about HD audio realization on PC and various other aspects in this problem. This filled numerous missing information links in pursuit for full-blooded and elegant HTPC solution aspects. Im think that this feature deserves much more attention, promotion and advertisment than got so far. This feature needs to be placed in products main feature list.

By the way I think that mass audio industry is stuck in 44.1 KHz, 2 Channel, 16bit sound for 28 years (since 1980) that is a shame !!!!

So far Im understand that first to HD audio solutins was inte nVidia joined it recently and AMD (my favorite :) ) still no solution for that :( , but Im think it will be to change as soon as AMD begins to feel pressure from rivals because of this lacking feature :).

The AACS spec (content protection for HD DVD and Blu-ray) mandates that any protected audio data on an "unprotected" interface be downconverted to at best 16-bit, 48kHz sound. Cyberlink has taken this considerably farther and downconverts everything-- protected and unprotected-- to 16-bit, 48kHz sound. I use alternative software like Media Player Classic or ZoomPlayer or even MediaPortal which is more difficult to configure but which can send out the full bit depth and sampling frequency up to 24-bit, 192kHz per channel.

Eventually it's likely PowerDVD will come around and allow full resolution over a protected interface, but the LPCM must be sent from the software to the HDMI port (inside the PC) encrypted or face this downconversion. The Intel HDMI Audio solution that exists on motherboards today will not meet this criteria, as the Intel HD Audio bus is not an encrypted interface. Future Intel chipsets will likely not have this limitation (don't know about the G45, which is coming Real Soon Now) because it will all happen on the chipset and not over an unprotected bus.

I had forgotten about the ADAT capabilities of some of Intel's motherboards, Mitch, so you raise a good point. While HDMI is superior, and seems the de facto standard, it was not the first to the party. In fact, there are some specialty sound cards which include 4 SPDIF ports and pass 2 LPCM channels per port to a receiver which is likewise equipped. Those solutions are pretty niche, though. ADAT is an actual standard, though mostly only used by audio professionals.