What kind of bluetooth gadget? Something you wear, like a watch? The pebble is a really ugly watch, so I can see you not wanting that.
So by bluetooth gadget do you mean something that sits on your desk? In your pocket? On your wrist? or something different? I would like something like the pebble watch, martian watch or even like the new samsung watches if they were nicer. I am hoping Apple does an iWatch, but makes it classy.
Unfortunately I'm not sure you're actually going to find anything like this for the iPhone. Apple doesn't allow apps (or bluetooth devices) to see if you have unread messages or how many unread messages you have.
Theoretically there could be a device that would work similar to the way the pebble does and it could notify you when you receive a new message, however, like the pebble that device would not know when you've read that message and wouldn't be able to turn off it's light indicating that you no longer have any new messages.