Here's brief summary of one hour on **#radeon** IRC channel:
- old DRM modules don't create /dev/dri/* for ATI R6xx/R7xx-cards, that's normal;
- kernel (up to, but not incl. 2.6.30) DRM aren't that old, but neither aren't new enough to support the cards in question;
- moreover, DRM mainline lacks their support too: therefore x11-base/x11-drm doesn't suit;
- in order to build DRM modules for R6xx/R7xx, one should git-clone source from specific git branch (see Xorg Wiki)
- use ebuild from x11 overlay with DRM_LIVE_BRANCH="r6xx-r7xx-support" added to your /etc/make.conf
Added later: described technique has no use, if you're running kernel 2.6.30 and later. Just compile DRM-modules and have fun.