This fork works for 'tags', not 'categories'.

jekyll-create-tag-pages-rakefile.rb

desc 'Generate tags page'
task :tags do
  puts "Generating tags..."
  require 'rubygems'
  require 'jekyll'
  include Jekyll::Filters
  
  options = Jekyll.configuration({})
  site = Jekyll::Site.new(options)
  site.read_posts('')
  site.tags.sort.each do |tag, posts|
    html = ''
    html << <<-HTML
---
layout: main
title: Posts tagged "#{tag}"
---
    <h1 id="#{tag}">Postings tagged "#{tag}"</h1>
    HTML

    posts.reverse.each do |post|
      post_data = post.to_liquid
      html << <<-HTML
        <div class="section list">
        <h1>#{date_to_string(post.date)}</h1>
        <p class="line">
          <a class="title" href="#{post.url}">#{post_data['title']}</a>
        </p>
        <p class="excerpt">
	  #{post_data['excerpt']}
        </p>
        </div>
      HTML
    end
    
    File.open("tags/#{tag}.html", 'w+') do |file|
      file.puts html
    end
  end
  puts 'Done.'
end

Or this one ?

desc 'Generate tags pages'
task :tags  => :tag_cloud do
  puts "Generating tags..."
  require 'rubygems'
  require 'jekyll'
  include Jekyll::Filters
  
  options = Jekyll.configuration({})
  site = Jekyll::Site.new(options)
  site.read_posts('')

  # Remove tags directory before regenerating
  FileUtils.rm_rf("tags")

  site.tags.sort.each do |tag, posts|
    html = <<-HTML
---
layout: default
title: "tagged: #{tag}"
syntax-highlighting: yes
---
  <h1 class="title">#{tag}</h1>
  {% for post in site.posts %}
		{% for tag in post.tags %}
			{%if tag == "#{tag}" %}
				{%include post.html%}
			{%endif%}
		{%endfor%}
  {% endfor %}
HTML

    FileUtils.mkdir_p("tags/#{tag}")
    File.open("tags/#{tag}/index.html", 'w+') do |file|
      file.puts html
    end
  end
  puts 'Done.'
end

desc 'Generate tags pages'
task :tag_cloud do
  puts 'Generating tag cloud...'
  require 'rubygems'
  require 'jekyll'
  include Jekyll::Filters

  options = Jekyll.configuration({})
  site = Jekyll::Site.new(options)
  site.read_posts('')

  html = ''
  max_count = site.tags.map{|t,p| p.count}.max
  site.tags.sort.each do |tag, posts|
    s = posts.count
    font_size = ((20 - 10.0*(max_count-s)/max_count)*2).to_i/2.0
    html << "<a href=\"/tags/#{tag.gsub(/ /,"%20")}\" title=\"Postings tagged #{tag}\" style=\"font-size: #{font_size}px; line-height:#{font_size}px\">#{tag}</a> "
  end
  File.open('_includes/tag_cloud.html', 'w+') do |file|
    file.puts html
  end
  puts 'Done.'
end