This blog post discusses has a small commandline tool (written by Nicholas Riley) to set the brightness from Terminal. Just download the binary [4KB] and install it. You can then set the brightness by typing brightness 0 (to set it to minimum, use 1 for maximum, or any value between).
Note: The binary is Intel-Only (32-Bit), but the source code is available on the above linked page too, so you could easily compile a PowerPC or 64Bit Intel Binary on your own. I've tested it only with Snow Leopard and a self-compiled 64Bit binary, and it works fine

