You probably remember this case: the “Peel” for iPod touch, which is basically a MiFi that clips onto your phone. Early reviews indicated it seemed to work pretty well, but it’s still not really an iPhone replacement.
Seems like your best bet in this case would be to get an actual MiFi and just use it when you need it. But if you absolutely must have the case, it’s available now for pre-order, and should ship starting in October. $119, though, that’s pretty steep. And no returns unless it’s DOA, so you better really want it.