Found this on the RubyGarden wiki and it totally made my day. To get persistent history rocking in IRB, add the following to your .irbrc.
require 'irb/completion'
require 'irb/ext/save-history'
ARGV.concat [ "--readline", "--prompt-mode", "simple" ]
IRB.conf[:SAVE_HISTORY] = 100
IRB.conf[:HISTORY_FILE] = "#{ENV['HOME']}/.irb-save-history"
Note that this will also add tab completion.
