dhenry:
- if you look at those datasheets for the devices, many of them will go down to 2-ish volt or even 1-ish volt for newer devices. So they don't have to work at 5v. The arduino happens to be designed to work with 5v. But if you were to jump that regulator, you can go down much lower than 5v. BTW, they don't need that 5v to be precise either.
If you do want to be precise, the ATmegaV328P microcontroller on the Uno has an operational range of 1.8 to 5.5 VDC. However, the operating voltage heavily influnces the clock speed. For example, a 16 Mhz clock is only really possible between 4.0 and 5.5 VDC (see Figure 29-145 of the datasheet). That's why ATmegaV328P-based boards operating on 3.3 VDC are usually only 8 Mhz (half the speed of the official Uno).
Edit: Just to be clear I am in agreement with dhenry's second point. 