Microcontroller with rotary encoder and bluetooth capabilities
Any small micro plus a cheap BT module should be suitable for this.  

For example, a PIC 12 or 16F, with something like this BT Module (I have used a surface mount version of one of these successfully in a recent prototype) plus the encoder could be thrown together very quickly for proof of concept purposes. On the PC side, Java would be best so you can port to Android, Mac, Linux, etc.