To play devils advocate....HTPCs are overated. I have one which I lovingly built as I'm a PC geek and I manage a computer store, but at least once a week i feel like chucking it out the window. If you haven't used one before, I'll make the following warnings
* The two most common HTPC OS's, MCE2005 and Vista Media Centre, can be quite infuriating. Apart from being PCs which crash now and then, the interface is really very simplistic for what you could imagine you could do with a PC and a remote control. Also any OS that requires you to do registry edits to get what should be standard functionality (e.g. FM tuner and DTV both working from the one tuner card) is pretty shoddy
* Unless you love fiddling and updating free software every month, you have to pay a third party a subscription to get the TV guide in media centre and unless you are in a larger metro area, some of the programming can be a little out of whack
* don't even think about getting FTA DTV and austar/foxtel running into your HTPC with working EPG unless you have some serious time and a lot of patience. the excellent xpmediacentre.com.au community is a godsend when attempting these tasks.
* HP has dumped it's entire media centre PC lineup. This shows their faith in a format they previously heavily supported and the reason it hasn't really taken off much at all apart from the enthusiast market is because they are not (relatively) simple, straightforward things like a set top box, DVD/HD recorder/player etc.
* There are two main selling points of a HTPC - Live TV recording/pausing etc via the EPG, and having your music collection available at the touch of a remote with albums/coverart/tracks/genres etc at your fingertips. The former can be tinged with frustration depending on how stable you get your EPG. The latter is the main reason I've stuck with my HTPC. I haven't got up to change a music CD in a year. Of course if you are an audiophile, you need to be very particular about the format your rip your music in and some would say any encoded format (heavily compressed or not) is going to affect the sound quality when listened to on good equipment. At the mo I still have sh*t equipment so it's not an issue
* WAF can plummet if you introduce a HTPC to the mix - your better half may not be so happy to reboot her TV, trying to find a wireless keyboard stashed in the living room as MCE has locked up and you can't close it without using the keyboard
On the positive side...
* If you are a geek and a PC hobbyist they can definitely offer some good fun and when configured right you can get the frustration levels down. As mentioned, the xpmediacentre.com.au community are brilliant, helpful, and being a part of that i almost as much fun as actually using your HTPC (again, if your a geek/PC hobyist)
* when set up and working the EPG is fab, set it to record every instance of a particular show, archive shows or start watching 30 mins behind live TV and fast forward the ads
In relation to your query - I've no idea about the new audio formats and HTPCs. I doubt there will be much of a push in that direction due to the relatively tiny HTPC market. Re HDMI, I suppose there is no HDMI output that carries audio+video on PCs because the video and audio are very seperate physical components, nearly always made by different companies, and trying to get them together to create a single HD output that carries audio and video would not be an easy (or inexpensive) task. You can get DVI to HDMI cables to send the video signal to your TV's HDMI port but there is no way to use the audio channel.
I'm also not too sure about the supposedly poorer SPDIF output quality to a receiver - most research i've done suggests there is negligible difference between optical and coax/spdif outputs. Again, no idea how or if this relates to any new HD audio format though (am too new to the audio side of things).
When i get my Yamaha receiver this week I will be connecting my HTPC to it via SPDIF/coax so will report back.
cheers
nat