http://developer.mbed.org/users/mbed_official/code/mbed/ - SDK?

http://developer.mbed.org/users/mbed_official/code/mbed-src/ - manually synced to:

https://github.com/mbedmicro/mbed - seems like official repo

http://developer.mbed.org/handbook/mbed-library-internals

https://github.com/lcvisser/mbed-lib