Not all iPhone Apps will work on your iPad for a simple reason – the tablet lacks a camera and the lower Wi-Fi only models don’t have GPS.
Some apps won’t make the iPad cut at all because the iPad lacks some features the iPhone boasts. Only the more expensive, 3G mobile broadband versions of the iPad will have global positioning system (GPS) functionality, for instance. The iPad is also missing a camera; an omission that could render some iPhone applications, like barcode scanners, useless.
To distinguish apps that will work on the different kinds of iPads, Apple has created software "flags," including a general "iPad Wi-Fi" flag and a more specific "iPad 3G" flag for apps that will only operate on the 3G-equipped units, says Kastelein. link.