The challenges are both technical & economic. As you said, there is the need for the wireless bandwidth to handle large numbers of users, but there is also the issue of the cost for them to use that bandwidth.
The concept of a very low-cost phone that relies heavily on the cloud is somewhat at odds with the availability & cost of using the cloud in developing nations. To some degree, it is an example of putting the cart before the horse.
Low cost is preferable to entire geographical areas and market segments in the overall industry. I don't know that we'll ever see sub 100 dollar PCs, or what that would look like exactly, but the demand for it would be there if someone could afford to make it.
I would think that as the cloud expands and develops the need for smartphones of a variety of calibers will also assert itself more and more. But that's not a real certainty. The cloud throws the entire smartphone market for a loop--it makes everything sort of questionable.
As the focus shifts to Cloud and a high spped network, there won't be any need for smart phones. The simple feature phones with a better display screen would be able to achieve the same perfromance what a smart phone of today would do.
So Firefox seems to have this vision while bringing their $25 phone.
The issue of the data plan cost has been one of the main discussions at the MWC. In most developing markets people use pre-paid cards, and data is very expensive to them. But most of them are using feature phones.
Facebook is working with Gemalto offiering basic FB experience on feature phones by special SIM cards with SMS packages. They want people to experience the social network and move to a smartphone, and carriers are happy to help to get new data customers. This model is already working in countries such as Morocco, Colombia and Chile.
But the data plan cost is not only an issue in developing markets. Here in Spain many people do not have a data plan for their smartphones, and try to connect only by wi-fi. Since basic free Wi-Fi is available in many cities they just jump from hotspot to hotspot and get a connection.
cloud is the right answer to fill this gap. Today we are building and maybe testing apps on laptop, PC or servers but what if all the processor load can be shifted to cloud servers and a developer can just write the app on tablet and present it.
The above statement may be true. However, the original idea of having low cost computer is not only for studying and information sharing but also for learning programming. As of today, neither smartphone nor any tablet devices is the right platform to do so. Most apps are indeed developed on a computer (either Mac or PC) and download it to the device for testing before submitted to the store.
To fill the gap of learning programming, maybe, there should be an apps that link to a virtual platform of a programming language. ;)