VIA recently announced that they have opened their reference documentation for their GPUs and are even now actively working with the openChrome project. For me, however, it's too little too late.
I've finally grown tired of even trying to get accelerated video working on my VIA-based MythTV box. XvMC support is simply non-existent and accelerated anything just doesn't work. With standard 480i broadcast TV I had no problem being CPU-bound for MPEG2 decoding, but it just doesn't fly with 720p and a pcHDTV card. I throw in the towel.
I'm looking at what Shuttle has to offer instead with either an Intel, NVIDIA or AMD platform. It appears that the graphic chipset choices break down into either GeForce 8, Intel GMA X4500HD, Intel GMA 3100 or GeForce 7050PV. The best NVIDIA choice appears to be the 7050PV, as it seems to enjoy known XvMC acceleration on MythTV's feature matrix and some have even reported getting it to work in a dual-head environment. The Intel cards should theoretically offer great performance for the power and enjoy good Linux driver support due to Intel's great contributions to X.Org, however Myth's wiki seems to know painfully little about the Intel GMA's. As far as XvMC support goes, Intel's chipsets don't seem to have a great track record.
So GeForce 7050 seems to be the most sane choice for those who are tired of fisticuffs with xorg.conf. But wait! We have audio to worry about too. If I'm going with a Shuttle GeForce 7050, then it looks like I'm going with a Realtek ALC888DD. Here again, MythTV notes that yet another Intel chipset is a true pain to work with. Still, I noticed that the MythTV hardware database notes there was at least one other Shuttle user with a similar setup that was able to get things to work.
Weirdly enough, both of the GeForce 7050 Shuttle boxes I found were AMD boxes. Go figure...