Before I aw this suggestion @Efonne:
If you boot from the LiveCD it should be possible to fix it by mounting your root filesystem and running this command that comes from /tmp/post_upgrade_command, adding to the paths as appropriate for where you mounted it:
tar xzpf /kernels/kernel_SMP.gz –exclude loader.conf -C /boot/
I had reinstalled 2.0.1.
I decided to try changing /boot/kernel/pfsense_kernel.txt to SMP and then upgrade to 2.1. If the upgrade didn't work I could try booting the LiveCD and issuing the tar command. The firmware upgrade completed successfully and the subsequent reboot successfully booted FreeBSD 8.3-PRERELEASE.