This is a tricky one. Bebook have been promising mobipocket support for a while. It was half-expected by July, but hasn't arrived yet.
Their homepage is now announcing that their next version will also have wifi. But mobipocket isn't mentioned.
http://mybebook.com/
You could contact them and ask. (Do tell us what their reply is.)
If mobipocket is what you want, then you might also consider a Bookeen Cybook Gen 3.