I found this snippet on the Railshacks blog:

# Array of plugins that you want to be reloaded on each request
reloadable_plugins = ["has_markup"]

# Remove the plugins from the load_once_paths variable
reloadable_plugins.each do |plugin_name|
  reloadable_path = RAILS_ROOT + "/vendor/plugins/#{plugin_name}/lib"
  Dependencies.load_once_paths.delete(reloadable_path)
end

Add this to config/environment.rb to allow your plugins to be reloaded on each new request:

config.reload_plugins = true if Rails.env == 'development'

See this commit for details.