It's not that simple. According to Apple itself, there's no i-Device yet supporting the Serial Port Profile (SPP) which is the only profile supported by most cheap bluetooth boards. So you either have to go with a more expensive bluetooth module that does support one of the Apple-supported profiles and work around the limitations that a HID profile has for example.
Better is probably to choose a WiFi shield or Bee to have a simple web server based interface. This way you don't need special software on the iPad and the WiFi stuff is in the same price category as the more elaborated bluetooth modules.