عمر دياب (aka Amr Diab) has artist samples on the profile page with Arabic spelling, so I assume that's how it's supposed to be tagged. Virtually every other Arab or Middle Eastern profile page I've seen on last.fm with artist samples is in English, for example; Nancy Ajram, Sherine, or Mostafa Amar.
So it doesn't seem like there's any agreed-upon way to tag Arabic tracks. My computer can read Arabic, but my mp3 player doesn't support non-Roman characters, so I can't read what I'm listening to when I use it, so I am trying to change all my tags to English.
Then the problem with transliteration becomes finding the "right" way to spell the Arabic titles, albums, and artist names. For example, Mostafa Amar is just one of the ways his name could be spelled; in fact, a proper non-shami pronunciation of his name would be "Mostafa Qamar", and his first name could be spelled like "Mustafa" or "Moustafa".