I'm having this same problem. Since the MPU9150 I'm using is embedded in epoxy, I can't test it the grounds.
I believe that the correct value for WHO_AM_I is the most significant 6 bits of the address which for either 0c68 or 0x69 would be 0x34.
Not sure why I keep getting 0x14...
This value is hard coded in MPU6050.cpp:
bool MPU6050::testConnection() {
return getDeviceID() == 0x34;
}