A small dongle (colored orange of course), about the size of Apple’s camera connection kit device, with a dock connector on the top. This device would serve as an ad-hoc connection to your Eye-Fi card allowing you to take photos and have them pop up on your iPad without the need of a WiFi network. There would be a software component as well that would serve as a large preview for the shots in real time, perhaps basic editing, and means of uploading or exporting to other apps.
I would see this device being both a stand alone purchase for users who already have Eye-Fi cards, or bundled with a card and available right at the Apple store.
(Yes, this could work with the iPhone as well)
(@alcedine helped me (did most of the work) with this little mockup!) :)