Hello Benj
I really was not paying due attention to pull up, but already had a partial success could read the HMC5883L magnetometer (break-out sparkfun) with software that you posted in,
http://www.matrixtsl.com/mmforums/viewt ... ass#p65640
changed the chip atmega 328p and the I2C port configured hardware channel 1. It ran well in the simulation and the real world.
Still not got success in reading the accelerometer and magnetometer LSM303 in the real world, only in the simulation, I believe being close to achieve.
maybe my problem is in the interruption or addressing.