Since the launch Apple has made efforts to hide some of the actual tech specs of the iPhone 4. Apple has never even hinted at what the processor speed or RAM is on this new iPhone. Apple seems to be trying to shield customers from these technical distractions and, instead, focusing on overall functionality.
The original iPhone and iPhone 3G contained 128MB of RAM, while when the iPhone 3GS was released it was boosted to 256MB of RAM. The iPad also contains 256MB of RAM. This discrepancy offers a technical explanation why Apple is not supporting iOS 4 features such as multi-tasking on the original and 3G iPhone.
The added RAM should help overall performance and multi-tasking on the new iPhone 4. This also explains why the iMovie App will only run on the iPhone 4. iMovie will only run on the iPhone 4 and not the 3GS or even the iPad.