Passing a template handler in the template name is deprecated.

Rails 開発 
render_to_string(
  :partial => "tags.html.erb",
  :locals => {
    :tags => @user_product.tags,
    :user_id => @login_user.id
  }
)

はDEPRECATION WARNINGが出る(Rails3.2.0.rc2)。

render_to_string(
  :partial => "tags",
  :formats => :html,
  :handlers => :erb,
  :locals => {
    :tags => @user_product.tags,
    :user_id => @login_user.id
  }
)

と書けばよい。