After lots of help from people on #debian on Freenode, and many dead ends explored, I found that setting the kernel option acpi=off makes the kernel (all kernels I've tested so far - 2.6.18, 2.6.21, 2.6.22 and a custom 2.6.22.5 from kernel.org) detect the DVD. So this works as a temporary solution. It would be nice to be able to have both DVD and ACPI working though.