Do not use Kernel 2.6.8
A patch that was introduced into the kernel shortly before the 2.6.8 release makes K3b and also the dvd+rw-tools unusable on Linux (unless run as root but that is not recommended). The very important GET CONFIGURATION MMC command is rejected by the kernel for reasons I cannot see and writing commands like MODE SELECT also fail (K3b cannot detect CD writers without it) even when the device is opened O_RDWR. Until this issue has been solved I strongly recommend to stick to kernel version 2.6.7.
Update: The kernel guys are currently fixing the problem so the next kernel release should work again. :)