Something related to ACPI seems to have changed since Debian Stretch update.
While Jessie was working fine with x60 libreboot, after update to Stretch the whole system hangs on switch to xserver after boot.
Must be related to ACPI, using acpi=off as a boot parameter makes system usable again
I can confirm this is happening to me as well, exactly as described.
Thinkpad X60s and libreboot latest release (20160907 dowloaded from https://vimuser.org/libreboot/). Debian Jessie works perfectly, whereas Stretch hangs before starting xserver (steady cursor on left side of the screen and black screen). Stretch boots correctly only in recovery mode without graphical interface and/or by adding the kernel parameter acpi=off at boot. In the latter case battery monitor and suspend to RAM are obviously not available. I really hope we will be able to use Stretch on this hardware in the near future, as this laptop is probably the easiest to flash with Libreboot and Jessie will be supported only a couple of years more.