Update: given the comments by WizenedEE below, I'm now of the feeling I shouldn't have accepted all of PaulS suggestions entirely as the gospel truth, although it sounds intuitively to make sense that many things change with a change in voltage, it seems that there are mechanisms specified to prevent this, such as crystals, and that there's no need to change fuses necessarily for a change in voltage. There still seems to some debate anyway. Now go on to read my original response >>>>>>>>>
With suitable changes to fuse settings, the Arduino can run at either 3.3V or 5V, not any random value in between.
Aha, makes perfect sense. I had no idea that fuses had to be changed to support a 3.3V set up, although I knew 8MHz operation needed a change in fuses I didn't figure for the voltage itself having to be accounted for. I guess the Jeenodes I had just came with the correct fuse settings out of the box. I'll follow up with JCW at Jeenodes and see if I can find out more about this compromise.
...bad things happen when the voltage is not constant. Serial data transmission, among other things relies on clock speed, which is a function of voltage.
Right, that makes a huge amount of sense too. Fortunately when we're connected to the USB (and wanting to attempt Serial comms) it will be powered off the 5V regulated. The rest of the time perhaps we'll get away with a slow clock (and incorrect timings). Once people have got the idea of programming simple behaviours and the arduino-compatible bug has bitten, we can expose them to more complex design issues, so when they start to design their own projects which demand a near-real-time clock or an actual real-time clock, then we can talk them through using a power regulator or a supporting chip. I think we're already stuck with a 'now we add 4 capacitors and a diode for protection' phase in the workshop, and this might be a moment to introduce the power regulator too.
Perhaps you're trying to underline my ignorance.
Not really. Just questioning why you think that the 5V is not important.
I simply didn't know why 5V was important. What you describe makes a lot of sense, thankyou.