Adafruit nRF8001 breakout board http://www.adafruit.com/product/1697 is now working with the FRDM KL25Z board. At least with the nRF Toolbox app and the iPhone 4S is working. I will do some more documentation in the next few days.

Just as I got my library just working (sort of) RedBearLabs have a posted a better working library. https://developer.mbed.org/teams/RedBearLab/code/BLE_nRF8001/ for the RedBearLabs shield. I've tested this with the simple controls example from the RedBearLabs app in iOS and it works well with the Adafruit board too. Especially good is being able to create characteristics on the fly without using the nRFgo program. Thanks to RedBearLabs and Sandeep Mistry who provided the setup info!

Connections

Wiring up the Adafruit nRF8001 breakout board.

Code

Pick one of the example files from RedBearLabs and configure the settings to match the connections to the mbed board.

The initialization of the peripheral will look something like this depending on which pins are used.

// The SPI construct, REQN and RDYN IO construct should be modified manually
// It depend on the board you are using and the REQN&RDYN configuration on BLE Shield
SPI spi(PTD2, PTD3, PTD1);      
DigitalInOut BLE_RDY(PTD5);  
DigitalInOut BLE_REQ(PTD0); 
DigitalInOut BLE_RESET(PTA13);  

Testing

Download the appropriate app from RedBearLabs. I've only tried the iOS app so far.