If i took a voltage regulator, or infact 2 12v or 5v positive regs.
then swapped 1 regulator output terminals (-/+), is that not -5 and +5 ? I'm thinking of running a computer from a 12v car battery using a load of switching positive regulators.... and to get all the negative rails reversing the outputs.....
No, it doesn't work that way.
You have to create a ground reference somewhere.
Perhaps if you fed 12V into one regulator, had it output +5V and call that FakeGround, used 12V- as the -5 output, and connected a 2nd regulator with 12V and FakeGround, use its output as +5.
Then you have +5, FakeGround, and -5.
+5 is more like 10V, FakeGround is 5V, and -5 is Gnd (or 12V return, likely the car chassis).
Then be sure to isolate the computer from any grounds in the car.
If you want the Arduino to share the same Ground as everything else you can use a DC-to-DC Converter to generate a -5V rail from the +5V or +12V rails.
If your 5V regulator gets hot you can probably get a single DC-to-DC converter that will provide +/-5V rails from the 12V rail.
Look on DigiKey.com or Mouser.com.
http://www.digikey.com/product-detail/en/RB-1505D/945-1439-5-ND/2303464
or something similar if you need more current
Search "recom" (brand),
then filter for 2 outputs, +5 & -5 outputs, go from there for input voltage, mounting style, etc.
Remember, raw car voltage can be nasty.
This one might be good, 9-18V input, +/-5V output, 500mA.
http://www.digikey.com/product-detail/en/REC5-1205DRW%2FH4%2FA/945-1472-5-ND/2318638
Computer tho - laptops generally only need 1 source, and generate their own voltage. Why do you need +/-5V?
trying to decide how i'd make a power supply that works from low voltage dc rather than rely on an ac 240 supply. i have over 10 regulators coming only + regulators.
my atx connection to the motherboard requir+12 -12v +5 -5v 3v so on, so it's going to be tricky then without some neg voltage regulators.
but virtual ground and what crossroads said is a useful and good starting point to learn from cheers (oh the car battery is not in the car 
but by me... )
Here's an ATX power supply for a car already designed and built:
And a cheaper on on eBay:
So why not simply get hold of an inverter (12DC to 230AC) and use standard ATX supply. Simple, cheap and the failure point is contained within the inverter.
ICL7660 IC is often used for this... if it's for low current.
