Wawa:
So buy an Arduino with onboard regulator, and then you don't use it.12volt on the DC socket is not a problem, unless you draw too much current from the 3.3volt or 5volt pins.
Leo..
So then you have to have an external regulator. It is usually a bad idea to have two 5V power supplies, particularly if you have any analogReads. So you have an Arduino with an onboard regulator which you don't use.