For those that need it, there's a tac(1) implementation at
http://www.feyrer.de/Misc/tac.tgz. I'll append the manpage below.
If enough people want this i guess it can be (re)considered for putting 
into NetBSD.


  - Hubert


TAC(1)                  NetBSD General Commands Manual                  TAC(1)

NAME
      tac -- concatenate and print lines of files in reverse order

SYNOPSIS
      tac [file ...]

DESCRIPTION
      The tac utility reads the named files sequentially, and writes their
      lines to the standard output in reverse order.  First all lines of the
      first file are printed, then the next files are handled.  If no filename
      is given, or if - is given as filename, tac will read from standard
      input.

SEE ALSO
      cat(1), sed(1)

HISTORY
      A tac utility first appeared in NetBSD 5.  The implementation based on
      sed(1) was done by Antti Kantee <pooka@NetBSD.org>, this manpage was
      written by Hubert Feyrer <hubertf@NetBSD.org>.

NetBSD 4.0_BETA2                March 27, 2007                NetBSD 4.0_BETA2