net_http_log.rb

require "net/http"

class Net::HTTP

  alias_method :request_without_log,  :request

  def request(request, body = nil, &block)
    time = Time.now
    response = request_without_log(request, body, &block)
    response
  ensure
    if self.started?
      url = "http#{"s" if self.use_ssl?}://#{self.address}:#{self.port}#{request.path}"
      ofset = Time.now - time
      rails_log("HTTP #{request.method} (%0.2fms)" % (ofset * 1000), url)
      rails_log("POST params", request.body) if request.is_a?(::Net::HTTP::Post)
      if defined?(response) && response
        rails_log("Response status", "#{response.class} (#{response.code})") 
        body = response.body
        rails_log("Response body", body) unless body.is_a?(Net::ReadAdapter)
      end
    end
  end

  def rails_log(message, dump)
    if defined?(Rails)
      Rails.logger.debug(format_log_entry(message, dump))
    end
  end

  def format_log_entry(message, dump = nil)
    if ActiveRecord::Base.colorize_logging
      message_color, dump_color = "4;32;1", "0;1"
      log_entry = "  \e[#{message_color}m#{message}\e[0m   "
      log_entry << "\e[#{dump_color}m%#{String === dump ? 's' : 'p'}\e[0m" % dump if dump
      log_entry
    else
      "%s  %s" % [message, dump]
    end
  end

end

1. Rails3 is ORM agnostic so ActiveRecord::Base may be not used

2. Deprecation:
   ruby-1.8.7-p330 :007 > ActiveRecord::Base.colorize_logging
   => "DEPRECATION WARNING: ActiveRecord::Base.colorize_logging and config.active_record.colorize_logging are deprecated. Please use Rails::LogSubscriber.colorize_logging or config.colorize_logging instead. (called from irb_binding at (irb):7)"

Use ActiveSupport::LogSubscriber.colorize_logging

3. Why Rails dependent?
   More generic:

# Usage:
#
#    require 'net/http/logging'
#
# == Class level
#
#    Net::HTTP.logger = Logger.new('/tmp/all.log')
#
# == Instance level
#
#     res = Net::HTTP.start(url.host, url.port) { |http|
#       http.logger = Logge.new('/tmp/instance.log')
#       http.request(req)
#     }
#     ...

# lib/net/http/logging.rb

require 'net/http'

module Net::HTTP::Logging
  class << self
    def self.included(base)
      base.send :extend, ClassMethods
      base.class_eval do
        alias_method :request_without_logging, :request
      end
    end
  end

  module ClassMethods
    attr_accessor :logger
  end

  attr_accessor :logger

  def initialize(*)
    super
    @logger = self.class.logger
  end

  def request(request, body = nil, &block)
    time = Time.now
    response = request_without_logging(request, body, &block)
    response
  ensure
    if self.started? && self.logger
    # ....
    end
  end

end

class Net::HTTP
  include Logging
end

if defined?(Rails.logger)
  Net::HTTP.logger = Rails.logger
end