After upgrading my T420s (https://forums.freebsd.org/threads/17308/page-5#post-324950) to FreeBSD-11-BETA2 I thought it would be worthwhile to share my findings. The process was mostly painless, except for the two points below. It should be noted that, on my machine, the power usage when idle improved from 14-15w to 12-13w, a noticeable advantage
1. Beware of the bootloader if using UEFI with ZFS
If you have just migrated to an UEFI system, like I have, when updating the bootloader after upgrading the ZFS pool, you are likely to make the same mistake that I did,
2. acpi_ibm requires flags for the sleep button to work
Add dev.acpi_ibm.0.handlerevents='0x04' to /etc/sysctl.conf
1. Beware of the bootloader if using UEFI with ZFS
If you have just migrated to an UEFI system, like I have, when updating the bootloader after upgrading the ZFS pool, you are likely to make the same mistake that I did,
gpart bootcode....
will happily render the system unbootable. The UEFI way is to copy the efi bootloader to the efi boot partition on the boot device, in my case, ada0 dd if=/boot/boot1.efifat of=/dev/ada0p1
2. acpi_ibm requires flags for the sleep button to work
Add dev.acpi_ibm.0.handlerevents='0x04' to /etc/sysctl.conf