xf86-video-via updated recently (thanks! :) ) but it was updated with a critical bug, easily fixed. Ebuild =x11-drivers/xf86-video-via-0.2.1 dies during startx with a "symbol 'assert' not found" and in dying it leaves the video card in a nonfunctional state. To fix this problem, add: #include <assert.h> at the top of the file src/via_dri.c. After that fix, everything works "perfectly". Someone with connections might push this upstream to the X11 via driver developer too.
Created attachment 84909 [details] Photograph of screen without assert.h I can confirm that. Without assert.h screen looks more or less like in the attachment (on Acer Apire 1362, K8N800 based laptop), with assert included works fine.
Please file this upstream at bugs.freedesktop.org in the xorg product. Make it block bug #5041, and post the URL here. Thanks!
Did a bit of searching and finally found this upstream so here's the url: https://bugs.freedesktop.org/show_bug.cgi?id=6668 Hope this helps as it seams this fix is still not in gentoo's portage.
It's not there since no one had brought it to our attention yet :D Thanks for finding it.
Fixed in CVS.