I need to design a mobile site for phones and tablet, should I design for retina and scale down or the opposite? Or should I even design for retina?
Apple was quite clever when they released the Retina display along with support at the OS level. They solved the problem of applications looking widely different on standard versus Retina displays.
This means that if you design the application without Retina awareness than you do not need to worry about it and it will simply work the same was as on a normal display.
If you do want media to take advantage of the Retina resolution, then you will need to use extensions to provide higher resolution media. Keep in mind that higher resolution also means higher bandwitdh which is often at a premium on mobile devices, so the extra resolution should only be used if it truly gives an advantage to your application. This may be the case for image application but for general websites, it is best left alone.