I know the data sheet says you really should have something like at least 3.6V Vin to power a 328p @ 16MHz, but I've got a little "pro mini" style clone board that seems to doing just fine running on 3.3V.
Anyone else have experience/thoughts/comments about running at 16MHz with only 3.3V? I'm finding this quite useful, in some applications it can simplify a lot of things, if it really turns out to be as solid as it seems...
ATmega's are overclocking-friendly, the undervolting of 0,3V should be no problem. I made some tests with that setting and all of these passed without problems. But make sure that you run your board under the worst case temp specs of atmel.
I have a view jeenode running 16@3.3 without a problem,
Take a look at
http://jeelabs.org/tag/jeenode/
The Seeeduino folks have in their standard 328p clone board a manual slide switch which can force the board to be powered either at 5 or 3.3 volts, both operating at 16Mhz, so I don't think you will see a problem exceeding the Atmel's datasheet speed Vs Vcc spec.
Lefty