I would assume that EllisLab would write their own from scratch as that seems to be their style but of course I would have no issue with them adopting Ion Auth.
In regards to Ion Auth with ajax and rest, I'm actually working on a project right now that uses Ion Auth in conjunction with ajax and rest for authentication but I won't be able to release any of the source code. Maybe I'll make some nice examples in a couple months when I have time...
