For carriage by aircraft, any package or magnet which has a magnetic field of more than 0.00525 gauss measured at 4.5 m (15 feet) from any surface of the package or magnet cannot fly.  If the magnetic field is less then 0.00525 gauss measured at 4.5 m (15 feet) then it is allowed in either checked or carry on.

Regulatory information: See the regulation: 49 CFR 173.21 (d)