Mark Stadler (mds@sgi.com) says:
  /dev/tport is a streams-based tty device driver which can be in one
  of 2 modes:

  - when X is not running, /dev/tport gets its input from the graphics
    keyboard and images in the frame buffer (textport mode).  This mode
    is only intended to be used in single-user mode or during
    transitional periods when the X server is not running.

  - when X is running, /dev/tport doesn't get any input and generates
    no output.  Any programs or shells using /dev/tport hang on reads
    and toss writes.

  Kind of a strange device.  But it makes more sense with a clear
  understanding of how /dev/console works.

  /dev/console is kind of like a terminal switch box.  Rather than
  switching physical rs232 cables, we direct console output onto the
  output stream of other streams-based devices who request such
  behavior with TIOCCONS ioctl.

  By default, /dev/console directs its output to /dev/tport.  In the
  absence of a windowing system this causes console output (including
  kernel prints) to show up on the textport in front of you.

  Once the window system is started, the /dev/tport is no longer
  visible.  Thus the console is no longer visible.  At this point, a
  terminal emulation window (using streams based ttys) can issue the
  TIOCCONS ioctl to cause console output (including kernel printfs) to
  show up in the emulation in the window in front of you.

  Note that when you bring the window system down, /dev/tport is still
  there with the same session it started with and the console output
  redirected back to it again.