It's an exciting idea, and it's streaks ahead of 'traditional' microwave transition atomic clocks.
These do not represent the state of the art, however, for which one should look at the experimentally demonstrated ~9e-18 accuracy by the Wineland group at NIST

http://arxiv.org/abs/0911.4527v2 ;

http://www.nist.gov/physlab/div847/grp10/ , or the Strontium ion clocks at NPL (Teddington, UK)
Essentially, the higher the frequency, the more clicks you get in a certain time, and the more accurate your clock can be (the smaller an error one missed click would represent). The caesium atomic clock is about 10 GHz (1E10 Hz). Strontium is in the optical, so a few 100THz (1E14). Aluminium ions are at about 1PHz (1E15 Hz). This new proposal with Thorium is around 7.6eV, which is about 2PHz, so not a million miles away from the current, demonstrated, state of the art.
Also... orbit of the neutron around the nucleus isn't a fair description of a magnetic dipole transition, which would more accurately be describes as a flip in the direction of the neutron's spin axis.

:)