@jareriks and everyone else,
Here are a few details regarding Bluetooth 4.0 (or BLE) modules and the status of various projects interfacing them with the Arduino. In my experience, there appear to be currently two main BLE modules being checked out in the Arduino community: the nRF8001 and the BLE112.
(1) The nRF8001
A couple of other hobbyists and I are working on interfacing Arduino with nRF8001 (software and breakout board). I particularly like the nRF8001 because it is a very low-cost ($4-5), tiny 5X5mm chip. Of course it has to be appended with some external circuitry like crystal, etc., but still I was able to do an Eagle layout where I fit the whole circuit within the equivalent area of a CR2032 coin cell – I really like the fact that I’d be able to make a wireless sensor so tiny and for a fairly low cost, while being able to communicate with my iPhone or Android.
We just yesterday created a github repo for the Arduino interface with nRF8001: https://github.com/nabilt/ArduinoBLE
Rigth now, it is empty, but in the next couple of weeks hopefully, the github page will contain both the Eagle schematics for a simple breakout board as well as the Arduino-library side of things, as progress ahppens. Everyone is welcome to contribute to this project; if you want to, just reply here or shoot me a PM, so you can be added.
(2) The BLE112
The Arduino interface with the second module, which is the BLE112, seems to be currently being worked on by Dr. Michael Kroll and independently by Jeff Rowberg (to see their pages, you can google either of their names along with BLE112). The BLE112 is a $15-16 module with an 18X12mm area. It’s larger and slightly more expensive, but I really like the fact that the module (which is based around TI’s CC2540 chip) is readily packaged, i.e., to interface with an external MCU, Arduino, etc without further part additions.
One more difference (for me, a minor one) is that BLE112 modules can be both host and slave whereas nRF8001 can be slave-only, but for my general hobby projects, slave-only on the Arduino side would be just fine.
Also, above, I mentioned COIN CELL in particular because Bluetooth LE of course seems to be targeted toward low-power, occasionally-transmitting-sensor-type applications, and I think the general cliche statement from these various manufacturing companies appears to be that the module can run off a coin cell for years (but of course, this should depend on the duty cycle). I think the point is that the BLE modules tend to be small in size and can run off coin cells which are usually designed for low current draw (although they allow slightly larger pulse discharge).
Here you can find the links to the datasheets for these two modules: