require 'rubygems'
Gem.manage_gems
def show_gem_rdoc
  puts "Your generated docs are all probably in #{File.join(Gem.dir, "doc")}"
  puts "Just to be safe, I'll print out every gem's RDoc location:"
  specifications_dir = File.join(Gem.dir, 'specifications')
  lacking_rdoc = []
  Gem::SourceIndex.from_installed_gems(specifications_dir).each do |path, spec|
    manager = Gem::DocManager.new(spec)
    if manager.rdoc_installed?
      doc_path = File.join(spec.installation_path, 'doc', spec.full_name)
      puts " #{spec.full_name} => #{doc_path}"
    else
      lacking_rdoc << spec.full_name
    end
  end
  unless lacking_rdoc.empty?
    puts "\nThese installed gems have no RDoc installed:"
    puts " #{lacking_rdoc.join("\n ")}"  
  end
end
show_gem_rdoc

Your generated RDoc is probably all in /usr/lib/ruby/gems/1.8/doc
Just to be safe, I'll print out every gem's RDoc location:
flexmock-0.1.7 => /usr/lib/ruby/gems/1.8/doc/flexmock-0.1.7
simple-rss-1.1 => /usr/lib/ruby/gems/1.8/doc/simple-rss-1.1
classifier-1.3.0 => /usr/lib/ruby/gems/1.8/doc/classifier-1.3.0
actionmailer-1.1.5 => /usr/lib/ruby/gems/1.8/doc/actionmailer-1.1.5
...

These installed gems have no RDoc installed:
Ruby-MemCache-0.0.1
RedCloth-3.0.4
sources-0.0.1
...

Your generated docs are all probably in /usr/lib/ruby/gems/1.8/doc
Just to be safe, I'll print out every gem's RDoc location:
 flexmock-0.1.7 => /usr/lib/ruby/gems/1.8/doc/flexmock-0.1.7
 simple-rss-1.1 => /usr/lib/ruby/gems/1.8/doc/simple-rss-1.1
 classifier-1.3.0 => /usr/lib/ruby/gems/1.8/doc/classifier-1.3.0
 actionmailer-1.1.5 => /usr/lib/ruby/gems/1.8/doc/actionmailer-1.1.5
 activesupport-1.3.0 => /usr/lib/ruby/gems/1.8/doc/activesupport-1.3.0
 midilib-0.8.5 => /usr/lib/ruby/gems/1.8/doc/midilib-0.8.5
 diff-lcs-1.1.2 => /usr/lib/ruby/gems/1.8/doc/diff-lcs-1.1.2
 rubyful_soup-1.0.4 => /usr/lib/ruby/gems/1.8/doc/rubyful_soup-1.0.4
 units-1.0.1 => /usr/lib/ruby/gems/1.8/doc/units-1.0.1
 acts_as_taggable-1.0.4 => /usr/lib/ruby/gems/1.8/doc/acts_as_taggable-1.0.4
 actionpack-1.11.2 => /usr/lib/ruby/gems/1.8/doc/actionpack-1.11.2
 needle-1.3.0 => /usr/lib/ruby/gems/1.8/doc/needle-1.3.0
 payment-1.0.1 => /usr/lib/ruby/gems/1.8/doc/payment-1.0.1
 pdf-writer-1.1.3 => /usr/lib/ruby/gems/1.8/doc/pdf-writer-1.1.3
 ruby-breakpoint-0.5.0 => /usr/lib/ruby/gems/1.8/doc/ruby-breakpoint-0.5.0
 facets-1.1.0 => /usr/lib/ruby/gems/1.8/doc/facets-1.1.0
 transaction-simple-1.3.0 => /usr/lib/ruby/gems/1.8/doc/transaction-simple-1.3.0
 rake-0.7.0 => /usr/lib/ruby/gems/1.8/doc/rake-0.7.0
 og-0.29.0 => /usr/lib/ruby/gems/1.8/doc/og-0.29.0
 activerecord-1.14.0 => /usr/lib/ruby/gems/1.8/doc/activerecord-1.14.0
 sparklines-0.2.6 => /usr/lib/ruby/gems/1.8/doc/sparklines-0.2.6
 color-tools-1.3.0 => /usr/lib/ruby/gems/1.8/doc/color-tools-1.3.0
 activesupport-1.2.5 => /usr/lib/ruby/gems/1.8/doc/activesupport-1.2.5
 cmdparse-2.0.0 => /usr/lib/ruby/gems/1.8/doc/cmdparse-2.0.0
 shipping-1.3.0 => /usr/lib/ruby/gems/1.8/doc/shipping-1.3.0
 htmltools-1.09 => /usr/lib/ruby/gems/1.8/doc/htmltools-1.09
 mime-types-1.15 => /usr/lib/ruby/gems/1.8/doc/mime-types-1.15
 fastercsv-0.1.9 => /usr/lib/ruby/gems/1.8/doc/fastercsv-0.1.9
 activerecord-1.13.2 => /usr/lib/ruby/gems/1.8/doc/activerecord-1.13.2
 net-ssh-1.0.8 => /usr/lib/ruby/gems/1.8/doc/net-ssh-1.0.8
 ferret-0.9.0 => /usr/lib/ruby/gems/1.8/doc/ferret-0.9.0
 ezcrypto-0.3 => /usr/lib/ruby/gems/1.8/doc/ezcrypto-0.3
 gruff-0.1.1 => /usr/lib/ruby/gems/1.8/doc/gruff-0.1.1
 BlueCloth-1.0.0 => /usr/lib/ruby/gems/1.8/doc/BlueCloth-1.0.0
 builder-2.0.0 => /usr/lib/ruby/gems/1.8/doc/builder-2.0.0
 creditcard-1.0 => /usr/lib/ruby/gems/1.8/doc/creditcard-1.0
 spreadsheet-excel-0.3.4 => /usr/lib/ruby/gems/1.8/doc/spreadsheet-excel-0.3.4
 glue-0.29.0 => /usr/lib/ruby/gems/1.8/doc/glue-0.29.0
 actionwebservice-1.0.0 => /usr/lib/ruby/gems/1.8/doc/actionwebservice-1.0.0
 io-reactor-0.05 => /usr/lib/ruby/gems/1.8/doc/io-reactor-0.05

These installed gems have no RDoc installed:
 Ruby-MemCache-0.0.1
 RedCloth-3.0.4
 RubyInline-3.4.0
 rails-1.0.0
 sources-0.0.1
 og-0.28.0
 facets_core-1.0.2
 SimpleSearch-0.5.0
 xml-simple-1.0.8
 facets-1.0.2
 facets_more-1.0.2
 facets-1.0.3
 GeoRuby-0.0.1
 postgres-0.7.1
 rubyzip-0.5.12
 stemmer-1.0.1
 glue-0.28.0
 libxml-ruby-0.3.6
 rmagick-1.10.1