"Ultrabook" is just a word that Intel trademarked like Intel did with "Pentium". That means Ultrabook usage will be governed by Intel to describe something. Intel will advertise the goodness of Ultrabook and allow any vendor who meets the Ultrabook specifications to use Ultrabook in the name or to describe their product. http://en.wikipedia.org/wiki/Ultrabook
There will not be an ARM "Ultrabook". There will not be an AMD "Ultrabook". Intel will not allow them to use the Intel trademarked name.
There will, however, be similar specifications from AMD and others with names that differ from Ultrabook.