Simple example of using Curses (built in to the ruby stdlib)

curses.rb

#!/usr/local/bin/ruby

require "curses"
include Curses

begin
  init_screen
  crmode
  x = 0
  y = 0

  setpos(x, y)
  addstr("Hit any key")
  refresh
  getch

  setpos(x, y+12)
  addstr("... AGAIN!")
  refresh
  getch

ensure
  close_screen
end

Curses clears the screen

These characters move the cursor up and clear the line:

print "\x1B[1A\x1B[2K" # move up and clear
print "\x1B[1A" # move up

These characters were found in the KarmaJS source (thanks Seb)

This article may be useful:
http://www.linuxselfhelp.com/howtos/Bash-Prompt/Bash-Prompt-HOWTO-6.html