A little more research. The book download is being blocked by Ingram Digital, the company responsible for the actual book download. fictionwise is just a retailer. fictionwise turns over the responsibility for delivering your book to you to Ingram Digital.
http://www.ingramdigital.com/
I don't know if other retailers use Ingram Digital or not. This could be a spat between fictionwise and ID or it could be an ID problem. If they are failing to deliver content for other retailers as well, this would seem to be an ID issue.
ID only services my MSReader books.
My books in mobipocket format are delivered by mobipocket.com.
Only some of the mobi ebook downloads are blocked.
An Evil Guest - Gene Wolfe/(Macmillan) - is Blocked
The Fabric of the Cosmos - Brian Greene/(Random House) is NOT blocked.