Jeremy had the answer, but the overall rule is:
don't change root's shell.

1) I tend to run "sudo zsh"  on machines I want root on.
2) the big danger is that your new shell is 
   a) not on the root partition
   b) can't get to any shared libraries it needs.


Quoting Nicholas Jackson (nicholasj@ncafe.com):
> I'm a NetBSD newbie and it appears that I've made a very stupid 
> mistake. I accidentally changed root's shell to an invalid shell using 
> chpass and now I'm unable to su to root.
> 
> I have the correct password and the system accepts it, however, when I 
> attempt to su to root, I am immediately dropped back into the unpriv. 
> user's shell with an 'invalid shell'. The log does show a momentary su 
> to root, though..
> 
> Is there something I can do? I'd hate to lose the work I've put into 
> this install..
> 
> Thank you!
> 
> Nicholas