mbed sdk is open source
I'm sure more than a few folks had some reservations about such a previously closed system.
For me, it'll be very educational with great examples on how to implement the mbed library of which I'm quite fond on LPC. They have extensive internals documentation posted up. I've scanned through it and I think it'll make a fascinating read.
It will likely open the door for porting to other Cortex M chips and platforms as well. Think LPC1769, STM32F4, Stellaris, etc.