mikejuk's submission paragraph states: "However, it's not all gain — the 3.3V operating voltage and the different I/O ports are going to create some compatibility problems. "
I respectfully disagree. Firstly, there are already a lot of 3.3v based Arduinos on the market. I own a JeeNode (see Jeelabs in EU, Modern Device in the USA). The JeeNode can run a 434MHz wireless radio transceiver and temperature sensor for MONTHS on a single 3.3v boosted AA battery. You could not do that with 5V.
Adafruit has a tutorial on converting Arduino Unos over to 3.3v, from 5v. It's popular.
Mostly all sensors these days are 3.3v.
But most most actuators (like stepper motors) require MORE than 5V. Sure, there's some relays requiring a mere 5v.. and very few work on 3.3v... but most relays require 6V or higher. The usefulness of 5V is diminishing, so what you really want is just enough power to activate a transistor or relay.
(Some Arduino compatible chips run great at 1.8v, and sensors do also... there will come a time someday where it may make sense to run at less than 3.3v)
I see Arduino more as a collection of standards and open hardware. There are dozens of Arduino designs all of which vary slightly in terms of electrical and physical (pinout, etc) compatibility. But this too is a good thing... the Arduino platform is all about ADAPTABILITY.