I want one. Badly. It soulds like Google has some really great ideas for such a device, and the ability to add some unique functions into a custom device is very intriguing. Imagine for instance, combining such diverse items as an RF power meter, a pulse oximeter, and a decent camera module in a device with extra battery and extra flash memory. It sounds like ARA is flexible enough to allow such a beast to be built.
I guess Google must have analyzed from the number of searches that people are interested to make their own phone. Its kind of achieving something. This is a very good move by Google. I am sure many were waiting for it.
If each module was embedded with an NFC-like comm chip to communicate between modules, it would truly be a revolution. I would most definitely install the RF transceiver module inside my shoe and way away from my brain. The days I don't need an oximeter; I will remove it from my pocket protector. Another module would reside in my wallet for transactions at Starbucks. The speaker modules would be embeded behind my ears. Since Google is working on corneal implants (a la Google Glass), the display would be contact lenses. The possibilities are endless. Woot!