If you are running an ATI Radeon card (I use a Radeon X1600 Pro) with the fglrx driver under Linux, and you're getting a black screen when attempting to start X, there may be a couple of kernel options that are set improperly. In my custom kernel build (18.104.22.168), I wound up enabling the CONFIG_MTRR_SANITIZER ("MTRR cleanup support") option and disabling the CONFIG_X86_PAT ("x86 PAT support") option. When I did those, all of a sudden, it worked. I don't know if those options are set this way in "stock" distro kernels, I'm just saying what I did.
After that, glxinfo reports accelerated video, fgl_glxgears runs at 700 fps, and the Second Life client for Linux runs successfully. It's taken me a long time to reach this point, so I hope someone else can benefit from my experience.