When I posted about this mare a little over a month ago, I said that I would pass along any further information.
The original seller (not linked) claimed that the mare was a "confirmed purebred", and it does appear that she was indeed registered. But since that time serious questions have been raised about her papers. It is a bit of a soap opera so I'll leave off the details, but discussions can be found here, here and here. (The sites linked may require registration.)
So for the moment there is a registered tobiano Arabian. She does not, however, appear to be a legitimate purebred. The registry is investigating, and most observers believe she'll lose her papers.
Note: Since this post was made, some of the linked discussions have been removed.