As JT says, they've been around for ages on freighters.
The sensors are located in the gear (I'm talking 744F here). The "problem" with this is that a perfectly flat base (ie ramp/taxiway/runway) is required to get a accurate reading. This of course is rare... The tolerance on the 744F is roughly 2%.
Considering the aircraft has a MTOW of close to 400T (more on the -ERF), you could have a difference of +-8T with this tolerance.
In our discussions with Airbus for the A380F, we brought up a request for a 1% tolerance. We were told this would be unaffordable.
Our guidline is that any difference of +-5T (again 744F) should be "investigated"
