Unknown at this point. They could allow you to just download the Mac client and use your existing account. I don't know why anyone would want to use the Mac client though, considering it's not a native client, it's going to have limited performance compared to playing it through bootcamp.

Also, it's not the same as the PlayStation/PC thing, considering Square can host the Mac client download themselves. For PlayStation you need to go through the PSN store because Sony wants it's share of the money.