Marvell is developing a module based on the PXA1928 CPU which was announced iN February, And Nvidia is working on a module based on the Tegra K1 CPU which has only shown up in a few high end tablets.
When combined with one of the 3 Project Ara base unit (as well as a screen, battery, and other components), the CPU modules will form the core of what may or may not be the next great shift in mobile devices.
In theory Project Ara will enable consumers to buy a base unit as well as components which met their needs: camera, screen, battery, wireless chip, etc. When all the parts arrive the consumer will be able to plug the parts into the base unit and have a working phone.
Or at least that is how it is supposed to work; whether the actual retail units will function adequately in the real world is another matter.
It's not clear at this time whether Google's hardware partners can design the mechanical connections to withstand removing a component multiple times or if the modules will stay attached during a day's normal use. (Sorry, I can't do Facetime right now; the camera fell out of my phone.)
But even though there are still unanswered questions, I'm still looking forward to the launch. I've never liked how unibody designs became the dominant form in the laptop market, making it difficult if not impossible to upgrade, so I am eager to see a new product that bucks the trend - even if it is designed to be a smartphone and not a laptop or tablet.
If the mechanics of the design work out then there are any number of places this can go. Developers have already started speculating on how Project Ara can be expanded. For example, earlier this year one developer posted an image of a concept design which would turn the Project Ara base unit into a handheld gaming device:
If I understand it correctly, that is based on one of the stock base units for Project Ara (the largest of the 3, I think) and adds a unique screen unit. If you can do that with an Ara then it shouldn't be too hard to also build a tablet or other mobile device.