In order for that to work, I think you necessarily need the cooperation of the network owner, and an informed end user. Given that, it seems like the technical solution is basic public key cryptography. Give the access point a public key (possible the same key it uses for https), then all the user has to do is validate that the access point is using the appropriate certificate.
You could provide software that automatically validates networks when you connect, and/or provides a more user friendly way to do so.
The biggest problem I see is that their is no way to automatically distinguish between a MITM, and the vendor simply not participating.
"informed end user." Or it could be a free automatic feature of Android 4.4. The almighty GOOG has the unusual situational advantage of being the same guy doing the WIFI and doing the phone/tablet OS. The only place AAPL has them beat is they sell the wifi access point devices too.
They could release an ITMS app for iDevices, I suppose.
My relatively new android phone is running 2.3.7, so somehow shoehorning it into an android app would be more useful than adding to a OS I probably won't have access to for years.
Its going to need to be cross platform for the laptop users anyway.
Note that as a startup idea you don't need to be .mil grade and encrypt and verify every packet. Something as simple as a notification pop up along the lines of "Holy Cow you are in Great Danger!" would be more useful than the present nothing.
The biggest problem I see is that their is no way to automatically distinguish between a MITM, and the vendor simply not participating.