It turns out I really DO need to call bus_space_unmap on the i2c_tag_t's
handle, or later uses of the i2c bus don't work.  So I had to add the
corresponding bootstrap hack (I think I got it right) in bus_space.c...
Actually, I'm pretty sure it's wrong, but it let the machine boot.

So I guess I was right to trust the spec.  :)

-s