When the iPhone 4 was announced, a lot of people were surprised that some features, like iMovie, were exclusive to iPhone 4. Not even the iPad, with its identical A4 processor, was said to be getting all the same apps.
However, it seems the issue is one of hardware, more specifically, RAM. MacRumors reports that while the original iPhone and iPhone 3G boasted 128MB of RAM and the 3GS and iPad had twice that with 256MB, the iPhone 4 has 512MB of RAM.
Though Apple isn't exactly known for releasing the processor speed and RAM of any of its iPhones, the company supposedly confirmed the 512MB at WWDC last week.