You should always be wary if any binary blobs are required (often the case with video drivers). They will limit what you can do in terms of upgrading the kernel and hence the rest of the OS. Sadly VIA have a history of not quite getting this whole "open" thing: http://www.phoronix.com/scan.php?page=article&item=via_o...
Although most end vendors of devices using the WM8xxx line of chips are GPL violators (direct-from-China business models where noone cares), Wondermedia themselves have been supplying GPL source via Harald Welte of Gpl-Violations (formerly also employed by VIA):
http://ftp.gpl-devices.org/pub/vendors/Wondermedia/
100% blob-free to my knowledge. Unfortunately they haven't released anything for the WM87xx series (apc.io) yet, but these devices are brand new so here's hoping.
If you're interested you can dig up some more details, and the follow from-scratch open kernel development efforts, at
http://groups.google.com/group/vt8500-wm8505-linux-kernel/
(What I -really- hope they release is publicly available datasheets.)