apretext:
Thanks for the link, I'll try and burn a 3.3v bootloader on to one of my chips and see if it works.
Or, as ShermanP clarified - an 8 Mhz bootloader.
If you choose 8 Mhz you can run at 3.3V or 5V with no issue and the chip won't care at all.
If you choose 16 Mhz you need 5V unless you wanna take your chances with reliability.
You might also want to change the brown out detection setting if you do end up running at 3.3V. The default BOD setting for UNO boards is 2.7V, which is a little close to the rail voltage if you end up at 3.3V. You could have accidental resets if you have dips in your supply V. Might consider BOD of 1.8V.