Migrate local paperclip assets to Amazon S3.

migrate_paperclip_to_s3.rake

##
# Use this behind the scenes to migrate files from your filesystem to Amazon S3
# %: rake paperclip_migration:migrate_to_s3
##

namespace :attachments do  
  desc "migrate files from filesystem to s3"
  task :migrate_to_s3 => :environment do
    require 'aws/s3'

    # Load credentials
    s3_options = YAML.load_file(File.join(Rails.root, 'config/s3.yml'))[Rails.env].symbolize_keys
    bucket = s3_options[:bucket]
    
    # Establish S3 connection
    s3_options.delete(:bucket)
    AWS::S3::Base.establish_connection!(s3_options)
    
    # determine the models to migrate
    klasses = [:article, :event, :user]
    
    klasses.each do |klass_sym|
      if @klass = real_klass(klass_sym)
        if @klass.respond_to?(:attachment_definitions) && definitions = @klass.attachment_definitions
          
          total_items  = @klass.count
          current_item = 1
          
          @klass.all.each do |record|
            definitions.each_pair do |column, value|
              attachment = record.send(column.to_sym)
              
              # NOTE: If the application is configured to use S3 already, 
              # calling exists? won't work as expected because paperclip will automatically look on Amazon.
              if !attachment.exists?
                total_items -= 1
                next
              end
              
              styles = value[:styles].keys rescue []
              styles.push(:original)
              
              styles.each do |style|
                
                # whatever mapping needed to where your files will live on S3
                # in my case, I changed the URL when migrating to S3
                path = attachment.url(style.to_sym).gsub('/system/files', '')
                file = attachment.to_file(style.to_sym)
                
                # skip if we already have this file on S3
                next if AWS::S3::S3Object.exists?(path, bucket)
                
                begin
                  print "uploading: #{style.to_sym}..."
                  AWS::S3::S3Object.store(path, file, bucket, :access => :public_read)
                  print " done.\n"
                rescue AWS::S3::NoSuchBucket => e
                  AWS::S3::Bucket.create(bucket)
                  retry
                rescue AWS::S3::ResponseError => e
                  raise
                end
              end
              
              # fancy output
              percent_complete = ((current_item.to_f / total_items.to_f) * 100).round(2)
              
              puts "Completed: #{current_item} / #{total_items}: ~#{percent_complete}%"
              
              current_item += 1
            end
          end
          
        else
          puts "There are no paperclip attachments defined for: #{@klass.to_s}"
        end
      else
        puts "#{klass_sym.to_s.classify} is not an existing model."
      end
    end
  end
  
  def real_klass(key)
    key.to_s.classify.constantize
  rescue
  end
end

Thanks, it's definitely pulled and inspired from a few other "migrators" floating around.
I've updated the gist with your recommendation. The app I was building this for only had image assets, but I've got others with non-image assets.