Neat idea using the magnets and phone's compass sensor; seeing it plugged into the headphone jack, I was expecting it to use the mic channel like the Square card reader.
LG/Google Nexus 4 support? It has a compass in it, so it should work right?
I can do some testing for you if you like.
I would be very surprised if the Google Nexus 4 does not work. As you say it has a compass in it... What is essential is that the compass has a high enough update frequency. All android phone we have tested so far (HTC Desire HD, Google Nexus S, GALAXY SII and GALAXY SIII) all have a good enough sensor. However we do not want to promise that it will work with a certain device before we have conducted a live "high speed test" up to what is equivalent to 25 m/s or 50 knots. Currently we have only fully tested the GALAXY SII and GALAXY SII.
As we are a small team we haven't had time to test more android phones.
Regarding the testing, we are trying to figure out how to best test a lot of android devices. It would be nice to 'just' send out a device to some lucky android owners, but we want to make sure that the devices is tested well enough at high speeds. (This can be done by using a fast drill that can rotate at around 30 Hz or 3600 RPM).