Sorry, took me a while.. had to compile the kernel and change my system to udev.
I can confirm this (bug?). Here (800MHz) while writing a DVD-RW it ran right the way down to 0%, then filled itself up again. On the one hand I think it's probably working ... as it never left 100% here before, but on the other hand 0%? Actually it did it twice while burning a 1.4GB file at 4x.

hdparm shows: using_dma = 1 (on)