For anyone interested, I found a solution to the problem described below.
I just cd'ed in /dev/ and ran ./MAKEDEV for each slice being used. I then
modified /etc/fstab with the correct slices for each filesystem and booted
under the new kernel. Everything has gone perfect after.
I had a similar nasty experience. It was made nastier by the fact that I
did a MAKEDEV and then booted without changing fstab -- MAKEDEV deleted my
wd0a /dev entries, putting me back in single user mode again! There seem
to be some transaction problems with the current arrangement, but they
largely have to do with slightly irregular slice configurations (such as
my config with two slices, only the a partition on the first, and a
failure to have the /dev entries).