irbをirb-0.9-02.09.03にしたら
% cat ~/.irbrc
require "irb-history"
require "irb/completion"
% irb
/usr/lib/ruby/1.6/irb/completion.rb:187: undefined method `completion_append_character=' for Readline:Module (NameError)
from /home/hishiura/.irbrc:2:in `require'
from /home/hishiura/.irbrc:2
from /usr/lib/ruby/1.6/irb/init.rb:198:in `load'
from /usr/lib/ruby/1.6/irb/init.rb:198:in `run_config'
from /usr/lib/ruby/1.6/irb/init.rb:196:in `each'
from /usr/lib/ruby/1.6/irb/init.rb:196:in `run_config'
from /usr/lib/ruby/1.6/irb/init.rb:195:in `catch'
from /usr/lib/ruby/1.6/irb/init.rb:195:in `run_config'
from /usr/lib/ruby/1.6/irb/init.rb:21:in `initialize'
from /usr/lib/ruby/1.6/irb.rb:55:in `start'
from /usr/bin/irb:13
となってirbが起動せんぞー。

補完って仕様変わったの？