I had the same issue as OP. Debian 8 (Jessie) live CD or USB would not boot on my T400 with libreboot (20150518), no matter what. I tried making the USB with both dd and unetbootin. I then tried the same CD and USB on a Thinkpad 520 with BIOS and it worked. On a whim I also tried a Linux Mint 17.3 USB (all my free software-loving bones quaked with horror...) on my T400 and it booted without any issues.
Something about how the Debian 8 iso is put together is incompatible with libreboot, no question. I really wanted Debian on my T400 with libreboot, so I ended up having to install Debian on my T520 then using dd to move the installation from the T520 to the T400. Ugh.
