Why do images shot at 300 ppi on a Nikon D300 display in PS CS5 at 240ppi at a larger physical size?
You should first understand that the concept of ppi is completely arbitrary. What matters is the total pixel count (horizontal x vertical). You do not shoot images at any particular ppi - the concept is meaningless.
But if you're converting raw files, what's causing the specific choice of a default ppi of 240 for you is the setting that you can adjust in Camera Raw, by following the link at the bottom-center of the dialog.
As Noel mentioned, cameras do not actually shoot with any ppi in "mind" but they can add the ppi info when they save the image file on the card in the instant after capturing the image if the file format ,such as jpg, supports ppi information. If you open such file in Photoshop the ppi will be respected. If you shoot using raw file format have in mind that it doesn't contain any ppi information. The ppi settings on your camera is applicable only if you shoot using a file format like jpg that can contain ppi information.