The Earth is certainly magnetic. Its magnetic field is what makes
North-South compasses align. The field is not nearly strong enough, or
different enough from place to place, to use in any sort of levitation
scheme. Even if it were stronger and had the right sort of spatial
variation, any levitation with magnets would be unstable. perhaps you
could actively adjust for that to stay levitated. With intense magnets
and superconducting materials, stable levitation can be obtained.
That's not practical for a skate board.
You'd also need a special surface, either made up of
superconductors or magnets (if you want to reduce the weight and
expense of the skateboard, you might want to make the underlying
surface out of electromagnets with special feedback systems which
supply the right push to keep the skateboard up).
I've seen a coil with a large alternating current flowing through
it levitating above a thick (about half an inch) aluminum sheet. Eddy
currents in the plate partially expelled the magnetic field from the
aluminum. The whole thing got hot very quickly, though, and it was
barely able to lift up the weight of the coil of wire, let alone a
skateboard with someone on it. It sounds expensive and impractical.
(republished on 08/02/06)