Weird...I can see two possibilities: either you had some process using 99% cpu that you didn't know of (yesterday), or some other user (root for example) was logged in at a console before you logged in to X. That way you won't get the correct permissions on /dev/dri/*, and 3d acceleration will fail.
Hmm...just for the record, I think you can forget the last one. I've mostly worked with Nvidia, where the permissions problem arises from the fact that if person A logs into X, then logs out, he or she usually leaves a few processes behind (gconfd and/or kdeinit), so that the next person who logs in won't get ownership of /dev/nvidia*. This does not happen with proper DRI drivers such as the XFree ATI ones, since X manages the permssions of /dev/dri/* based on Section "DRI" in XF86Config.
Anyway, just thought I'd mention that. Another possibility would of course be that another user was simultaneously logged into X, using another server.
/Peter
