reverse_deps.rb

require 'rubygems'

fetcher = Gem::SpecFetcher.fetcher

my_dep = 'activesupport'

# 1st arg = all (true to get all versions, false to get latest)
# 2nd arg = prerelase (true to get pre-release gems, false to get stable) 
all_gems = fetcher.list(false, false)

my_deps = []

all_gems.each do |uri, gems|
  gems.each do |gem|
    begin
      spec = fetcher.fetch_spec gem, uri
      my_deps << [gem, uri] if (spec.runtime_dependencies + spec.dependencies).any? {|d| d.name == my_dep}
    ensure
      # urr something borked with this gemspec
    end
  end
end

require 'rubygems/commands/query_command'

Gem::Commands::QueryCommand.new.send(:output_query_results, my_deps)