You need to have read/write permissions to /dev/dri/cardX to benefit from 3D hardware acceleration in Xorg X Server. On a Gentoo linux machine, this file has the following permissions set by default :

ls -l /dev/dri/card0 
crw-rw---- 1 root video 226, 0 2009-10-14 16:12 /dev/dri/card0