Can you try to do this:
sysctl hw.acpi.handle_reboot=1
You just solved my issue with the latest snapshot! (2.0-RC1 (i386) built on Thu May 5 06:05:22 EDT 2011)
It hangs after message "Rebooting..." but executing this in commmand prompt solved this.
I use KVM for VM emulation.
UPDATE: uuups, seems still doesn't work

UPDATE2:
It doesn't work becouse adding this to (new created) file /boot/loader.conf.local doesn't work
same when I add this to /boot/loader.conf
just take a look - have to add manually (screenshot)
UPDATE3:
still hanging on "Rebooting...", even with Executed command and handle reboot enabled.
UPDATE4:
added to /etc/sysctl.conf
SOLVED: Guess what? The problem was I was using QEMU domain type instead of KVM! Checked this on different server (I'm using AMD Phenom while different server has Core i5 and has no problem). Simple change in XML of VM and problem solved.