Ok i do more research and 3d working now. This is what i find:
file /lib/firmware/radeon/R600_rlc.bin is missing from kernel26-firmware.
This file can only be loaded if you use kms late start
- To use early start
(http://wiki.archlinux.org/index.php/ATI#Early_start) the file should be included in initramfs ... but i really dont know how ...
My earlier attempts at getting kms were flawed due to having nouveau-drm and xf86-video-nouveau installed. I have a notebook with an nvidia card and another one with an ati card, the usb disk I use for emergency recovery/testing was supposed to be general but it seems it can't be, anyway, I have removed that now and (late) kms does start, however radeon_ucode from aur is needed if I want to have dri and xv working with my RV610 (radeon hd2400). With a X600XT (r300) everything works fine without installing radeon_ucode.
Sorry for the earlier noise about kms not working. The need for radeon_ucode for some cards should be advertised with an install message (at least until aur packages are no longer needed to get everything working as well as possible imho).