Tobster86:
Perhaps I'm not sure then!
(see below...)
Tobster86:
Specs for the Arduino indicate a recommended input voltage range of 7-12V; surely the best I could do is a (presumably rare) 7V regulator, then throwing away 2V with the Arduino's regulator?
7V is no problem. LM317 regulators are adjustable to any voltage you want.
...but are we going to convert 17V to heat?? That's even worse than converting 12V to heat. 
Why not use a switching regulator? (like the ones I linked to above) They don't deliberately convert power to heat, they transform it.
If you don't want to buy on eBay then somebody like Pololu will be happy to charge you more: Pololu - Step-Up/Step-Down Voltage Regulators