While you could do that, it may be simpler to just get a bluetooth shield or board and use it without having to mod something else. Note, there are many different bluetooth profiles, and if you hack apart a device, the bluechip inside may be hardwired to do just keyboard, headset, etc. I imagine you would also need to provide an antenna, which is likely part of the design of the device, and not easily removed.
For example, I bought this shield made by seeed at my local Radio Shack for my Uno R3: http://www.radioshack.com/product/index.jsp?productId=12688459
. There is a pin that controls whether the communication is on pins 0/1 for use with the hardware serial port or pins 6/7 for use with software serial ports.
And I bought the following to use on my Teensy 3.0 that runs at 3.3 volts: http://www.ebay.com/itm/HC-05-Bluetooth-Transceiver-Host-Slave-Master-Module-Wireless-Serial-6pin-/221158958927?pt=LH_DefaultDomain_0&hash=item337e19e74f
. Now, it claims that it can handle 3.3-6 volts, so in theory you could use it on the Uno/Mega/Leonardo without a voltage converter, but I haven't tried it yet. On the teensy 3.0, you need to hook it up to one of the 3 serial ports (0/1, 7/8, or 9/10). If you look for HC-05 or HC-06 which are common bluetooth chips, you can find various bluetooth devices on places like ebay. If you are ordering it, make sure it is a complete set, where somethings what is offered for sale is the actual chip that you might have to solder connectors to, or just the plastic substrate. Also, some of the cheaper versions are slave only or master only, and unless you know precisely what you want, it may be good to pay a little more for one that can do both master and slave.
Both of these run fine on their respective platforms, and I can connect to them via my Android smart phone using the Bluetooth Controller or Connection Terminal apps on the smart phone. They implement a serial text protocol, and you can use print/println to write text to the bluetooth stream, and available/read to read text from it. They each have different ways to switch from master to slave mode, setting the name, connecting, etc.