MAS3:
If you're thinking about a mobile setup with motors, have a look at BEC speedcontrollers as used in RC models.
These often have a regulated power output to the RC receiver, and if i remember correctly, these are supposed to be at 4,8 or 6 volts.
Might be worth to have a look at these.
Why use a ESC with BEC to power the 'Duino when OP does not need to use a brushless motor ?
You can get standalone BEC's
To OP: I would recommend a board like this anytime when powering the 'Duino from a voltage higher than 5,5V
Much more effecient, no extra heat!
// Per.