Can I use a retina display at actual pixel size?
Yes you can, but you probably will not choose to do so. When you really want to force the display into 2880x1800 you can easily configure it that way. Here are four of the better options available today to do what you ask:


SwitchResX
Retina DisplayMenu (used to be SetResX)
scrutil
ChangeResolution


That said, items like the menu bar with the Apple icon are extremely small at 2880x1800 and you don't need the entire screen to at that high resolution for a properly build application to let the windows it controls have the full resolution. The scaling modes are pretty good, and you can always reduce your font sizes even while running at the doubled 1440x900 resolution. Most people will never need to set 2880x1800 since they really only care that the applications they use get the retina treatment and don't need to force the rest of the UI to run with such a dense packing of pixels.