I asked Thrangu Rinpoche this question. He replied, all who recite Amitabha's mantra are reborn in Sukhavati, and that certainly includes arhats. Also, when arhats are awoken from nirvana by the blessing of Amitabha, they are reborn in Sukhavati.
Having obtained the supreme freedoms
and conjunctions of the precious human existence,
endowed with faith, energy, and intelligence,
Having attended on a worthy spiritual friend
and received the pith of the holy instructions,
May we practice these properly, just as we have received them,
without obstacle or interruption.
In all our lives, may we practice and enjoy the holy dharma.