OMG... how many people are having problems with this... i must write tutorial for this.
Set this as the last layer and when all is done just delete it and compile your game.
eg. for portrait mode:
Set project window and layout sizes to 640:960 and basicly set everything important (that needs to be seen) into the smalest screen aspect ratio (iphone4) and make sure that your background covers both biger aspects (iphone 5 and ipad) and your good to go... and spawn everything that needs to enter screen from outside those bigger aspect ratios!
(if you are more expirienced than you can use anchors to stretch visible objects to fit into every aspect ratio)
use scale outer and set in your Layout "unbounded scrolling" to YES.